Refactor ResponseCache usage

Refactor store.have_events

It turns out that most of the time we were calling have_events, we were only
using half of the result. Replace have_events with have_seen_events and
get_rejection_reasons, so that we can see what's going on a bit more clearly.

Avoid creating events with huge numbers of prev_events

In most cases, we limit the number of prev_events for a given event to 10
events. This fixes a particular code path which created events with huge
numbers of prev_events.

fix federation_domain_whitelist

we were checking the wrong server_name on inbound requests

Update canonicaljson dependency

This reverts commit 9fbe70a7.

It turns out that sortedcontainers.SortedDict is not an exact match for
blist.sorteddict; in particular, `popitem()` removes things from the opposite
end of the dict.

This is trivial to fix, but I want to add some unit tests, and potentially some
more thought about it, before we do so.

1.1.2 was a bit broken too :/

Turns out that ObservableDeferred.observe doesn't return a deferred if the
result is already completed. Fix handling and improve documentation.

1.1.0 and 1.1.1 were broken, so we're updating this to help people make sure
they don't end up on a broken version.

Also, 1.1.0 is speedier...

Adds a `.wrap` method to ResponseCache which wraps up the boilerplate of a
(get, set) pair, and then use it throughout the codebase.

This will be largely non-functional, but does include the following functional
changes:

* federation_server.on_context_state_request: drops use of _server_linearizer
  which looked redundant and could cause incorrect cache misses by yielding
  between the get and the set.
* RoomListHandler.get_remote_public_room_list(): fixes logcontext leaks
* the wrap function includes some logging. I'm hoping this won't be too noisy
  on production.

Add metrics for ResponseCache

Add metrics for event processing lag

Document the behaviour of ResponseCache

Send events to ASes concurrently

Synapse on PyPy

Merge commit '11d2609d' of https://github.com/matrix-org/synapse into release-v0.27.0