Stop the master relaying USER_SYNC for other workers (#7318)

Long story short: if we're handling presence on the current worker, we shouldn't be sending USER_SYNC commands over replication.

In an attempt to figure out what is going on here, I ended up refactoring some bits of the presencehandler code, so the first 4 commits here are non-functional refactors to move this code slightly closer to sanity. (There's still plenty to do here :/). Suggest reviewing individual commits.

Fixes (I hope) #7257.

synapse/api/constants.py

@@ -97,6 +97,8 @@ class EventTypes(object):
 
     Retention = "m.room.retention"
 
+    Presence = "m.presence"
+
 
 class RejectedReason(object):
     AUTH_ERROR = "auth_error"