#8567 started a span for every background process. This is good as it means all Synapse code that gets run should be in a span (unless in the sentinel logging context), but it means we generate about 15x the number of spans as we did previously.

This PR attempts to reduce that number by a) not starting one for send commands to Redis, and b) deferring starting background processes until after we're sure they're necessary.

I don't really know how much this will help.

This can happen if e.g. the room invited into is no longer on the
server (or if all users left the room).

* Limit AS transactions to 100 events

* Update changelog.d/8606.feature

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

* Add tests

* Update synapse/appservice/scheduler.py

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

This should (theoretically) allow for using the TCP code with a different output type
and make it easier to use the JSON code with files / console.

We need to make sure we are readu for the `set_cache_factor` callback.

* Add `DeferredCache.get_immediate` method

A bunch of things that are currently calling `DeferredCache.get` are only
really interested in the result if it's completed. We can optimise and simplify
this case.

* Remove unused 'default' parameter to DeferredCache.get()

* another get_immediate instance

rather than have everything that instantiates an LruCache manage metrics
separately, have LruCache do it itself.

This implements a more standard API for instantiating a homeserver and
moves some of the dependency injection into the test suite.

More concretely this stops using `setattr` on all `kwargs` passed to `HomeServer`.

Optionally sends typing, presence, and read receipt information to appservices.

These are tests for #8439

* Fix outbound federaion with multiple event persisters.

We incorrectly notified federation senders that the minimum persisted
stream position had advanced when we got an `RDATA` from an event
persister.

Notifying of federation senders already correctly happens in the
notifier, so we just delete the offending line.

* Change some interfaces to use RoomStreamToken.

By enforcing use of `RoomStreamTokens` we make it less likely that
people pass in random ints that they got from somewhere random.

Update `EventCreationHandler.create_event` to accept an auth_events param, and
use it in `_locally_reject_invite` instead of reinventing the wheel.

Support returning a new event dict from `check_event_allowed`.

this is always the same as requester.access_token_id.

Should fix #3365.

Currently background proccesses stream the events stream use the "minimum persisted position" (i.e. `get_current_token()`) rather than the vector clock style tokens. This is broadly fine as it doesn't matter if the background processes lag a small amount. However, in extreme cases (i.e. SyTests) where we only write to one event persister the background processes will never make progress.

This PR changes it so that the `MultiWriterIDGenerator` keeps the current position of a given instance as up to date as possible (i.e using the latest token it sees if its not in the process of persisting anything), and then periodically announces that over replication. This then allows the "minimum persisted position" to advance, albeit with a small lag.

This could, very occasionally, cause:

```
tests.test_visibility.FilterEventsForServerTestCase.test_large_room
===============================================================================
[ERROR]
Traceback (most recent call last):
  File "/src/tests/rest/media/v1/test_media_storage.py", line 86, in test_ensure_media_is_in_local_cache
    self.wait_on_thread(x)
  File "/src/tests/unittest.py", line 296, in wait_on_thread
    self.reactor.advance(0.01)
  File "/src/.tox/py35/lib/python3.5/site-packages/twisted/internet/task.py", line 826, in advance
    self._sortCalls()
  File "/src/.tox/py35/lib/python3.5/site-packages/twisted/internet/task.py", line 787, in _sortCalls
    self.calls.sort(key=lambda a: a.getTime())
builtins.ValueError: list modified during sort

tests.rest.media.v1.test_media_storage.MediaStorageTests.test_ensure_media_is_in_local_cache
```

This PR allows Synapse modules making use of the `ModuleApi` to create and send non-membership events into a room. This can useful to have modules send messages, or change power levels in a room etc. Note that they must send event through a user that's already in the room.

The non-membership event limitation is currently arbitrary, as it's another chunk of work and not necessary at the moment.

All handlers now available via get_*_handler() methods on the HomeServer.

We call `_update_stream_positions_table_txn` a lot, which is an UPSERT
that can conflict in `REPEATABLE READ` isolation level. Instead of doing
a transaction consisting of a single query we may as well run it outside
of a transaction.

We call `_update_stream_positions_table_txn` a lot, which is an UPSERT
that can conflict in `REPEATABLE READ` isolation level. Instead of doing
a transaction consisting of a single query we may as well run it outside
of a transaction.

This allows a user to store an offline device on the server and
then restore it at a subsequent login.

Lots of different module apis is not easy to maintain.

Rather than adding yet another ModuleApi(hs, hs.get_auth_handler()) incantation, first add an hs.get_module_api() method and use it where possible.

* Optimise and test state fetching for 3p event rules

Getting all the events at once is much more efficient than getting them
individually

* Test that 3p event rules can modify events