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`.

The logic for marking invites as locally rejected was all well and good, but
didn't happen when the remote server returned a 500, or wasn't reachable, or
had no DNS, or whatever.

Just expand the except clause to catch everything.

Fixes https://github.com/matrix-org/synapse/issues/761.

inline `reject_remote_invite`, which only existed to make tracing the callflow
more difficult.

The documentation on get_json has been wrong ever since the very first commit
to synapse...

Also:
 - change the REST endpoint to have a "S" on the end (so it's now /read_markers)
 - change the content of the m.read_up_to event to have the key "event_id" instead of "marker".

Update README.rst

The FreeBSD port has been moved to the net-im category

Implement Read Marker API

Reduce federation replication traffic

Add a counter metric for successfully-sent transactions

Dedupe KeyedEdu and Devices federation repl traffic

Remove HTTP replication APIs