Add performance counters for different stages of loading events

Conflicts:
	synapse/app/homeserver.py
	synapse/state.py

During room intial sync, only calculate current state once.

Transaction counters

txn names

Need to use re.search if looking for matches not at the start of the string. Also comparisons with None should be 'is'.

Profiling

create identicons for new users by default as default avatars