Fix a small race condition when looking up the ROOT region.
Fix a small race condition when looking up the ROOT region.

There was a check-and-act sequence on the ZooKeeper object
without acquiring the appropriate lock, which means that
we could potentially cause a NullPointerException while
retrying to lookup ROOT.

Also move the ZooKeeper callback into a real class instead
of an anonymous class.

OpenTSDB/asynchbase,SteamShon/asynchbase,pythian/asynchbase,hhshuai/asynchbase,Connexity/asynchbase,OpenTSDB/asynchbase,Connexity/asynchbase,tempbottle/asynchbase,mway08/asynchbase,angushe/asynchbase,manolama/asynchbase,mapr/asynchbase,yuzhu712/asynchbase,nvoron23/asynchbase,mapr/asynchbase,hhshuai/asynchbase,tempbottle/asynchbase,tsuna/asynchbase,SteamShon/asynchbase,manolama/asyncbigtable,adityakishore/asynchbase,adityakishore/asynchbase,angushe/asynchbase,OpenTSDB/asyncbigtable,yuzhu712/asynchbase,manolama/asynchbase,mway08/asynchbase,tsuna/asynchbase,nvoron23/asynchbase