GC does not delete WAL files belonging to dead tservers

The GarbageCollectWriteAheadLogs uses a Map<HostAndPort,Long> to track when it had first seen a dead tserver, waiting an hour before deleting the files. However, a new instance of this class is re-created during each run of the SimpleGarbageCollector, causing the state of the dead tservers to be lost. All of the WAL files belonging to a dead tserver will never be removed.