Ensure good comprehension hygiene using flake8-comprehensions.

... and use it in places where it's trivial to do so.

This will make it easier to pass room versions into the FrozenEvent
constructors.

... to make way for a forthcoming get_room_version which returns a RoomVersion
object.

Fixes a bug where rejected events were persisted with the wrong state group.

Also fixes an occasional internal-server-error when receiving events over
federation which are rejected and (possibly because they are
backwards-extremities) have no prev_group.

Fixes #6289.

Python will return a tuple whether there are parentheses around the returned values or not.

I'm just sick of my editor complaining about this all over the place :)

Migrate all tests to use the dict-based config format instead of hanging items off HomeserverConfig (#5171)

Collect all the things that make room-versions different to one another into
one place, so that it's easier to define new room versions.

We need power levels for this test to do what it is supposed to do.

* 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.

Changes to m.room.power_levels events are supposed to be handled at a high
priority; however a typo meant that the relevant bit of code was never
executed, so they were handled just like any other state change - which meant
that a bad person could cause room state changes by forking the graph from a
point in history when they were allowed to do so.

Construct the EventContext in the state handler rather than constructing one and then immediately calling state_handler.annotate_context_with_state

Remove annotate_event_with_state as nothing was using it. Update state tests to call annotate_context_with_state