The aim here is to get to a stage where we have a `PersistEventStore` that holds all the write methods used during event persistence, so that we can take that class out of the `DataStore` mixin and instansiate it separately. This will allow us to instansiate it on processes other than master, while also ensuring it is only available on processes that are configured to write to events stream.

This is a bit of an architectural change, where we end up with multiple classes per data store (rather than one per data store we have now). We end up having:

1. Storage classes that provide high level APIs that can talk to multiple data stores.
2. Data store modules that consist of classes that must point at the same database instance.
3. Classes in a data store that can be instantiated on processes depending on config.

Before all streams were only written to from master, so only master needed to respond to `REPLICATE` commands.

Before all instances wrote to the cache invalidation stream, but didn't respond to `REPLICATE`. This was a bug, which could lead to missed rows from cache invalidation stream if an instance is restarted, however all the caches would be empty in that case so it wasn't a problem.

Proactively send out `POSITION` commands (as if we had just received a `REPLICATE`) when we connect to Redis. This is important as other instances won't notice we've connected to issue a `REPLICATE` command (unlike for direct TCP connections). This is only currently an issue if master process reconnects without restarting (if it restarts then it won't have written anything and so other instances probably won't have missed anything). 

* release-v1.13.0:
  Don't UPGRADE database rows
  RST indenting
  Put rollback instructions in upgrade notes
  Fix changelog typo
  Oh yeah, RST
  Absolute URL it is then
  Fix upgrade notes link
  Provide summary of upgrade issues in changelog. Fix )
  Move next version notes from changelog to upgrade notes
  Changelog fixes
  1.13.0rc1
  Documentation on setting up redis (#7446)
  Rework UI Auth session validation for registration (#7455)
  Fix errors from malformed log line (#7454)
  Drop support for redis.dbid (#7450)

Be less strict about validation of UI authentication sessions during
registration to match client expecations.

Since we only use pubsub, the dbid is irrelevant.

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.