dumper: fix Objecter locking

Locking expectations changed at some point, and the Dumper wasn't
updated to comply:
1) We need to take the lock for Objecter, as it
doesn't do so on its own any more.
2) We need to drop the lock in several places so that Objecter
can take delivery of messages

Signed-off-by: Greg Farnum <greg@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>