server: Fixed race-condition between deleting a volume (which involves...

server: Fixed race-condition between deleting a volume (which involves unregistering it at the DIR) and the periodic registration of volumes by the HeartbeatThread.

New methods pauseOperation() and resumeOperation() where introduced in the HeartbeatThread which can be used by other threads to synchronize with this thread. The DeleteVolumeOperation was changed acoordingly.

This fixes issue 278.