Byte-range locks of expired clients not ignored

[kofemann]

As resources allocated by expired clients are reclaimed by a periodically running cleanup thread, there are situations when a lock by a valid client denied due to conflicting lock of an expired client (pynfs test COUR2 testLockSleepLock).

This should be fixed in AbstractLockManager#lock method by checking , that conflict lock is held by a valid client.