Convert onTransactionIdle() callers to DeferredUpdate subclasses

Convert onTransactionIdle() callers to DeferredUpdate subclasses

* This puts the complex logic here after the commit step for
  all DBs, making the main multi-DB transaction more likely
  to be atomic. Previously, the idle callbacks could be hit
  bewteen DB commits.
* Enforce transactionality via AtomicSectionUpdate.
* Use $this instead of $that hacks for old PHP versions.

Change-Id: Idf7d54fdac6487f86907099680f5c1c4f5530b4e