[TACHYON-220] WIP Comments describing issues and possible solutions 

Some initial comments to help improve the performance of master info. From what I gather there is locking on stack local objects that are never really put back into the map. I would appreciate suggestions before implementing it, hence the WIP tag.
I'm not sure we need to be changing any of these maps to ConcurrentHashMaps right now. Right now, all of these maps are called inside a synchronized (mRootLock) or synchcronized (mWorkers) or some other type of synchronized block, so nobody will be running concurrent operations on them. Maybe we keep these as regular HashMaps until we figure out how to refactor the MasterInfo to remove those synchronized blocks.