Speed up filtering of a single event in push

Fix invite state to always include all events

print something legible if synapse already running

web_client_location documentation fix

When creating a new table index in the background, guard against it existing already. Fixes
https://github.com/matrix-org/synapse/issues/2135.

Also, make sure we restore the autocommit flag when we're done, otherwise we
get more failures from other operations later on. Fixes
https://github.com/matrix-org/synapse/issues/1890 (hopefully).

Add some extra logging for edge cases of federation

Check that requested room_id exists

Make state caches cache in ascii

* document how to make IPv6 work

* spell out that pip will install 17.1 by default

Reduce size of joined_user cache

setting up metrics, just adding/clarifying 2 very minor items

The _get_joined_users_from_context cache stores a mapping from user_id
to avatar_url and display_name. Instead of storing those in a dict,
store them in a namedtuple as that uses much less memory.

We also try converting the string to ascii to further reduce the size.

Reduce cache size by not storing deferreds

Fix code for reporting old verify keys in synapse

Currently the cache descriptors store deferreds rather than raw values,
this is a simple way of triggering only one database hit and sharing the
result if two callers attempt to get the same value.

However, there are a few caches that simply store a mapping from string
to string (or int). These caches can have a large number of entries,
under the assumption that each entry is small. However, the size of a
deferred (specifically the size of ObservableDeferred) is signigicantly
larger than that of the raw value, 2kb vs 32b.

This PR therefore changes the cache descriptors to store the raw values
rather than the deferreds.

As a side effect cached storage function now either return a deferred or
the actual value, as the cached list decriptor already does. This is
fine as we always end up just yield'ing on the returned value
eventually, which handles that case correctly.

Fix rejection of invites to unreachable servers

Only intern ascii strings

Remove unused cache

I haven't (yet) documented all of the user-list APIs introduced in
https://github.com/matrix-org/synapse/pull/1784 because the API shape seems
very odd, given the functionality.

When we're rejecting invites, ignore the backoff data, so that we have a better
chance of not getting the room out of sync.

The `except SynapseError` clauses were pointless because the wrapped functions
would never throw a `SynapseError` (they either throw a `CodeMessageException`
or a `RuntimeError`).

The `except CodeMessageException` is now also pointless because the caller
treats all exceptions equally, so we may as well just throw the
`CodeMessageException`.