librados: refactor watch/notify; return notify error code

librados: refactor watch/notify; return notify error code

Get rid of a level of intermediate classes with confusing names and put
the notify and notify finish logic in a single place so that it is easier
to follow and understand.

Pass the return value from the notify completion message to the caller.

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