Remove $recursionGuard var from tryOpportunisticExecute()

Remove $recursionGuard var from tryOpportunisticExecute()

Aside from there being no idle callback setting here, the
old addUpdate() code that runs updates allows nesting updates.
Make this support that for a few transitional commits. It will
be changed later to simply order the sub-updates after their
parent updates, keeping both outer scope for all updates and
locality of related updates.

Change-Id: I0ad4e9713a7893b981b7bb013e9db803eed663b2