Addressing review feedback.

Fix LLInstanceTracker::key_iter constructor param; accepting
InstanceMap::iterator by non-const reference relied on Microsoft extension
that accepts non-const reference to an rvalue. Given typical iterator
implementation, simply accept by value instead, which makes gcc happy too.

It turned out impossible to properly remove an inventory item
from within LLDiscardAgentOffer::done(), because that would lead to
nested LLInventoryModel::notifyObservers() calls.

Fixed that by deferring removal until the next LLAppViewer::idle() iteration.

Also elimiteed duplicated code.