Specifically, if currently_active remains true then we should not notify
if only the last active time changes.

* Add infrastructure to the presence handler to track sync requests in external processes

* Expire stale entries for dead external processes

* Add an http endpoint for making users as syncing

Add some docstrings and comments.

* Fixes

Squash-merge of PR #345 from daniel/anonymousevents

Remove users from the remote_offline_serials list (and clean up empty elements) when they go online again

Store a list of the presence serial number at which remote users went offline, so that when we delete them from the cachemap, we can still synthesize OFFLINE events for them (SYN-261)

setup_test_homeserver function in utils.

Rename MockedDatastoreTestCase to MockedDatastorePresenceTestCase since it is still presence-specific

shareable

Fix an off-by-one bug in presence event stream pagination; this might be responsible for any number of bug reports

SYN-103: Ignore the 'origin' key in received EDUs. Instead take the origin from the transaction itself