Optimised FilteredChildIterator and PlugIterator.

They were making unnecessary temporary IntrusivesPtr, which perform unnecessary reference counting operations, which have unnecessary overhead.

This alone gives more than a 5% speedup in a simple Instancer hash benchmark because the CompoundPlug uses a ValuePlugIterator in its hash implementation. Similar speedups should be seen in other areas.
Merge pull request #862 from johnhaddon/iteratorOptimisation

Optimised FilteredChildIterator and PlugIterator.