Use a stream id generator for backfilled ids

Add concurrently_execute function

Fix the invalidation of the names and aliases cache

Remove outlier parameter from compute_event_context

Use event.internal_metadata.is_outlier instead.

Remove the is_new_state argument to persist event.

Move the checks for whether an event is new state inside persist
event itself.

This was harder than expected because there wasn't enough information
passed to persist event to correctly handle invites from remote servers
for new rooms.

Split out RoomMemberHandler

Add replication streams for ex outliers and current state resets

Don't ignore the obey overlay if the rule has an enabled attribute of False

Fixes https://github.com/vector-im/vector-web/issues/1244

Add a replication stream for state groups

Use a stream id generator to assign state group ids

Require user to have left room to forget room

This dramatically simplifies the forget API code - in particular it no
longer generates a leave event.