Update docker runtime image to Alpine v3.11

Add dummy_events_threshold which allows configuring the number of forward extremities a room needs for Synapse to send forward extremities in it.

For the record, the reason we need this is as follows:

each RDATA command comes down the redis pipe as a subscription message. txredisapi as written needs at least three reactor ticks to read each subscription message from the tcp buffer. Hence, once the process gets loaded, it starts getting behind, and eventually redis knifes the connection. it then takes ages for the master to work its way through the backlog, before it reconnects again, during which any commands from any workers are dropped.

Make get_e2e_cross_signing_key delegate to get_e2e_cross_signing_keys_bulk

... mostly because the latter has a cache.

There's no point carefully dividing a list into batches, and then completely
ignoring the batches.

use an upsert to update device_lists_outbound_last_success

most of these params don't really need to be lists.

Speed up fetching device lists changes in sync.

Fix lost events on replication reconnection

... otherwise we can believe we're up to date when we're not.

Currently we copy `users_who_share_room` needlessly about three times,
which is expensive when the set is large (which it can easily be).

We're pretty close to having mypy working for `synapse.federation`, so let's
finish the job.

This caused `prev_state_ids` to be incorrect if the state event was not
replacing an existing state entry.

looks like we managed to break this during the refactorathon.

This will be used to coordinate stream IDs across multiple writers.

Functions as the equivalent of both `StreamIdGenerator` and
`SlavedIdTracker`.

Fix MANIFEST.in