Use static JSONEncoders

json encoders have an encode method, not a dumps method.

Use simplejson throughout

using json.dumps with custom options requires us to create a new JSONEncoder on
each call. It's more efficient to create one upfront and reuse it.

Let's use simplejson rather than json, for consistency.

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