Fixed error logging in db module
Added queries for working on sharded group state
ShardedUser heartbeats its presence periodically
Added user presence in group heartbeats

Renamed methods and queries that handle session/state
Added threaded observers for meta/state

Added group meta/state getters

Added base t.w.s user/group/realm code to integrate DB
Added meta changefeed update
Credentials checker checks session activity
Lookup properly identifies local, active remote, inactive remote users.
Merge pull request #104 from kzvezdarov/sharded-user

Sharded user