Signed-off-by: Silke Hofstra <silke@slxh.eu>

It seemed to suffer from a bunch of off-by-one errors.

* Split state group persist into seperate storage func

* Add per database engine code for state group id gen

* Move store_state_group to StateReadStore

This allows other workers to use it, and so resolve state.

* Hook up store_state_group

* Fix tests

* Rename _store_mult_state_groups_txn

* Rename StateGroupReadStore

* Remove redundant _have_persisted_state_group_txn

* Update comments

* Comment compute_event_context

* Set start val for state_group_id_seq

... otherwise we try to recreate old state groups

* Update comments

* Don't store state for outliers

* Update comment

* Update docstring as state groups are ints

We extract the storage-independent bits of the state group resolution out to a
separate functiom, and stick it in a new handler, in preparation for its use
from the storage layer.

... instead of creating our own special SQLiteMemoryDbPool, whose purpose was a
bit of a mystery.

For some reason this makes one of the tests run slightly slower, so bump the
sleep(). Sorry.

Configure the connectionpool used for unit tests to run the `on_new_connection`
function.

This isn't used, and looks thoroughly bitrotted.

Add federation_domain_whitelist

gives a way to restrict which domains your HS is allowed to federate with.
useful mainly for gracefully preventing a private but internet-connected HS from trying to federate to the wider public Matrix network

Extracted from https://github.com/matrix-org/synapse/pull/2820

Twisted core doesn't have a general purpose one, so we need to write one
ourselves.

Features:
- All writing happens in background thread
- Supports both push and pull producers
- Push producers get paused if the consumer falls behind

It turns out that the only thing we use the __dict__ of LoggingContext for is
`request`, and given we create lots of LoggingContexts and then copy them every
time we do a db transaction or log line, using the __dict__ seems a bit
redundant. Let's try to optimise things by making the request attribute
explicit.

pointless function is pointless

Fix flakiness in the UTs caused by the user_directory being updated in the
background

https://github.com/matrix-org/synapse/pull/2755 broke log-config generation,
which in turn broke the unit tests.

Instead of returning False when auth is incomplete, throw an exception which
can be caught with a wrapper.