Fix Resolver shutdown behaviour

* Add Resolver prefix to logging.
* Fix AsyncObject::removeReference() logic.
* Fix work remaining logic.

I spotted this while playing around with Application Stoppable behaviour. 
Reviewers: @nikb @vinniefalco 
In that case could quite easily put the whole body of signalled in the lambda? One thing I'm not certain of is if m_journal will still be active in the case of error being boost::asio::error::operation_aborted, which indicates that m_signals has been destructed. I believe this could be theoretically possible with some very rare timing combination of `rippled stop` followed by a SIGINT after Application has been destructed. Perhaps it should log to std::cerr instead?