Attempt to fix one of several USDImaging Nested Instancing Update Bugs

[murphyeoin]

Description of Change(s)

This is an attempted fix for https://groups.google.com/forum/#!topic/usd-interest/YPK05ebe8x4.
The most pared down examples of this are here:https://gist.github.com/murphyeoin/03637aaa39ced964e0d27cd207cc0b00

• Disappointlingly, this does not fix either of the repros from this issue
• I only understand a small amount of the code, so this fix is designed to minimise side-effects
• we are testing it in production now (definitely needs some more work - currently getting Coding Error: in _ResyncInstancer at line 1213 of PLACEHOLDER/scratch/dev/AL_USD/usdImaging/pxr/usdImaging/lib/usdImaging/instanceAdapter.cpp – Failed verification: ' instIt != _instancerData.end() '
  in some circumstances)

Fixes Issue(s)