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

Fixes https://github.com/matrix-org/synapse/issues/2431

Adds config option `encryption_enabled_by_default_for_room_type`, which determines whether encryption should be enabled with the default encryption algorithm in private or public rooms upon creation. Whether the room is private or public is decided based upon the room creation preset that is used.

Part of this PR is also pulling out all of the individual instances of `m.megolm.v1.aes-sha2` into a constant variable to eliminate typos ala https://github.com/matrix-org/synapse/pull/7637

Based on #7637

It looks like `user_device_resync` was ignoring cross-signing keys from the results received from the remote server. This patch fixes this, by processing these keys using the same process `_handle_signing_key_updates` does (and effectively factor that part out of that function).

The idea here is that if an instance persists an event via the replication HTTP API it can return before we receive that event over replication, which can lead to races where code assumes that persisting an event immediately updates various caches (e.g. current state of the room).

Most of Synapse doesn't hit such races, so we don't do the waiting automagically, instead we do so where necessary to avoid unnecessary delays. We may decide to change our minds here if it turns out there are a lot of subtle races going on.

People probably want to look at this commit by commit.

When a call to `user_device_resync` fails, we don't currently mark the remote user's device list as out of sync, nor do we retry to sync it.

https://github.com/matrix-org/synapse/pull/6776 introduced some code infrastructure to mark device lists as stale/out of sync.

This commit uses that code infrastructure to mark device lists as out of sync if processing an incoming device list update makes the device handler realise that the device list is out of sync, but we can't resync right now.

It also adds a looping call to retry all failed resync every 30s. This shouldn't cause too much spam in the logs as this commit also removes the "Failed to handle device list update for..." warning logs when catching `NotRetryingDestination`.

Fixes #7418 

... 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.

and associated functions:
 * on_receive_pdu
 * handle_queued_pdus
 * get_missing_events_for_pdu

The intention here is to make it clearer which fields we can expect to be
populated when: notably, that the _event_type etc aren't used for the
synchronous impl of EventContext.

This is in preparation to refactor FrozenEvent to support different
event formats for different room versions

This test didn't do what it claimed to do, and what it claimed to do was the
same as test_cant_hide_direct_ancestors anyway.

This stuff is tested by sytest anyway.

when processing incoming transactions, it can be hard to see what's going on,
because we process a bunch of stuff in parallel, and because we may end up
recursively working our way through a chain of three or four events.

This commit creates a way to use logcontexts to add the relevant event ids to
the log lines.