The StatusThread.balanceTablets method could loop indefinitely
in some cases, like when a TabletServer dies but is the target
of a current migration. The status of the TabletServers is
checked before this method is invoked and never updated. Created
a new property to indicate how many iterations should pass before
the Tablet Server status is re-checked.

Modified code such that only migrations for the current data level
would be considered and to only recheck tablet server status for
the root and metadata levels. Balancing of tablets returned for
non-current data level will be processed in their data level.