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 
@rec No. Example: `handleVerifyTimer` has a name, you can search for it, and it can be documented at the point of declaration:
https://github.com/ripple/rippled/blob/develop/src/ripple/overlay/impl/PeerImp.cpp#L100