librados: add rados_watch_flush() call

Add a call so that callers can make sure all queued callbacks have
completed before shutting down the ioctx.  This avoids a segv triggered
by the LibRadosWatchNotifyPPTests/LibRadosWatchNotifyPP.WatchNotify2Timeout/1
test due to the ioctx being destroyed when the in-progress callback
does a notify_ack.

Signed-off-by: Sage Weil <sage@redhat.com>
librados: watch_flush() on shutdown

Users can easily forget this. It makes shutdown potentially block, but if
they have racing callbacks they get what they ask for.

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