Rados performance: fixed missing unwatch call and added instrumentation.

The missing unwatch fix should improve performance of watch/notify based locking significantly.
Instrumentation will log any call to rados longer that 1s to /var/tmp/cta-rados-slow-calls.log.
Also prepared a structure to allow switching between watch/notify and backoff based locking.
Backoff code is not yet brought back (will test with the unwatch fix first).