Merge branch 'erikj/fixup_rejecting_invites' of github.com:matrix-org/synapse into erikj/require_format_version

Currently they're stored as non-outliers even though the server isn't in
the room, which can be problematic in places where the code assumes it
has the state for all non outlier events.

In particular, there is an edge case where persisting the leave event
triggers a state resolution, which requires looking up the room version
from state. Since the server doesn't have the state, this causes an
exception to be thrown.

When we add new event format we'll need to know the event format or room
version when parsing events.

We also implement `make_membership_event` converting the returned
room version to an event format version.

Add metric for user dir current event stream position

Move SRV magic into an Agent-like thing

In the debian package, make the virtualenv symlink python to /usr/bin/python3.X
rather than /usr/bin/python3. Also make sure we depend on the right python3.x
package.

This might help a bit with subtle failures when people install a package from
the wrong distro (https://github.com/matrix-org/synapse/issues/4431).

Currently we only have the one event format version defined, but this
adds the necessary infrastructure to persist and fetch the format
versions alongside the events.

We specify the format version rather than the room version as:

1. We don't necessarily know the room version, existing events may be
   either v1 or v2.
2. We'd need to be careful to prevent/handle correctly if different
   events in the same room reported to be of different versions, which
   sounds annoying.

* Remove unnecessary ORDER BY clause

* Add logging

* Newsfile

this makes it easier to stub things out for tests.

it is only ever a bytes now, so let's enforce that.

this thing is now redundant.

... instead of the matrix_federation_endpoint

Pull the magic that is currently in matrix_federation_endpoint and friends into
an agent-like thing

Fix bug when removing duplicate rows from user_ips

Config option to disable requesting MSISDN on registration

This was caused by accidentally overwritting a `last_seen` variable
in a for loop, causing the wrong value to be written to the progress
table. The result of which was that we didn't scan sections of the table
when searching for duplicates, and so some duplicates did not get
deleted.

All this magic is redundant.