As we want to have it run on the main synapse instance

The intention was for the check to be called as early as possible in the
request, but actually was called just before the main ratelimit check,
so was fairly pointless.

 Split event creation into a separate handler

* 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

Remove redundant return value from _calculate_state_delta

Factor out common code for search insert

we already have the state from _get_new_state_after_events, so returning it
from _calculate_state_delta is just confusing.

we can reuse the same code as is used for event insert, for doing the
background index population.

... as a precursor to making event storing and doing the bg update share some
code.

montoring metrics for number of cache evictions

Ensure media is in local cache before thumbnailing

Handle url_previews with no content-type

Factor out resolve_state_groups to a separate handler

factor _get_new_state_after_events out of _calculate_state_delta

Docstring fixes

Add an admin route to get all the media in a room

avoid failing with an exception if the remote server doesn't give us a
Content-Type header.

Also, clean up the exception handling a bit.

Fix a couple of errors in docstrings

The return type was a complete lie, so fix it

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.

(to make way for a method that actually just does the state group resolution)

Fix sql error in quarantine_media

Signed-off-by: Travis Ralston <travpc@gmail.com>

This reduces the scope of a bunch of variables

Remove spurious log argument

Remove unused "event_type" param on state.get_current_state_ids