[Database] Fixed case where trx idle callbacks might be lost.

[Database] Fixed case where trx idle callbacks might be lost.

* (bug 41656) LoadBalancer always commits if there are callbacks pending.
  This works by checking a new DatabaseBase::writesOrCallbacksPending() function.
* Made transaction idle callbacks run in autocommit mode. Generally callers already
  want autocommit mode or do quick begin()/commit() calls anyway. The docs already
  make stat that callbacks should close any connections they start, but this makes
  it harder for people to forget about implicit transactions. Since the callbacks
  often may happen in commitMasterChanges(), if transactions are left open, they
  might not get committed by anything afterwards.
* Added sanity exceptions and warnings if callbacks get lost for some reason.
* Renamed trxIdleCallbacks -> mTrxIdleCallbacks for consistency.

Change-Id: I00e1e0fcdd7deeee1fbac6d0f295160479cb8962