Send party_id on events and check the party_id of incoming events matches
Includes a basic test to assert that it actually does: we should
build out a decent test suite for calls as there's a lot of edge-case
functionality that can break and slip through the cracks (eg. glare).
This is a start.
Fixes https://github.com/matrix-org/matrix-js-sdk/issues/1511
Start the migration to v1 VoIP by supporting m.call.reject, which
we'll send if the caller says they're v1. Our version stays as v0
for now, until we speak the rest of v1.
Honour the default reaosn in a hangup being user_hangup.
Use the new local-age field for deciding whether a call is still
valid or not. Also add a grace period so we don't ring half a second
before the call becomes invalid.
