librados: unit tests for mon_command

Signed-off-by: Sage Weil <sage@inktank.com>
librados: add rados_monitor_log()

Add method to trail the cluster/monitor log for use by ceph -w.  Add a
simple functoinal test.

Note that changing the subscription level does not currently work
due to the protocol; you can't cancel a subscriptions.

Signed-off-by: Sage Weil <sage@inktank.com>