Measure time it takes to calculate state group ID

Fix error when claiming e2e keys from offline servers

fix tests/storage/test_user_directory.py

Linearize calls to _generate_user_id

Remove last usage of ujson

Add counter metrics for calculating state delta

Make sure we stringify any exceptions we return from keys/query and keys/claim,
to avoid a 'not JSON serializable' error later

Fixes #3010

this stuff is badly c&p'ed

Add Cache-Control headers to all JSON APIs

This will allow us to measure how often we calculate state deltas in
event persistence that we would have been able to calculate at the same
time we calculated the state for the event.

404 correctly on missing paths via NoResource

fixes https://github.com/matrix-org/synapse/issues/2043 and https://github.com/matrix-org/synapse/issues/2029

Use .iter* to avoid copies in StateHandler

It is especially important that sync requests don't get cached, as if a
sync returns the same token given then the client will call sync with
the same parameters again. If the previous response was cached it will
get reused, resulting in the client tight looping making the same
request and never making any progress.

In general, clients will expect to get up to date data when requesting
APIs, and so its safer to do a blanket no cache policy than only
whitelisting APIs that we know will break things if they get cached.

Fix replication after switch to simplejson

rearrange ordering of releases to match chronology