Disable transaction warnings for automatic trx.

Disable transaction warnings for automatic trx.

This tracks which transaction was opened automatically because of the
DBO_TRX flag, and then supresses any warnings about committing these
transactions implicitely, even if write operations were performed by
that transaction. To get warnings about implicitely committed writes
from automatic transactions, enable $wgDebugDBTransactions.

This change is a follow-up to I1e746322 and the older I3f1dd5fd, and
should be considered an alternative to I3eacc5a9. The new warning
behavior is:

* when beginning a transaction, warn if there is a transaction pending
that has also been started explicitely by calling begin().

* when beginning a transaction and $wgDebugDBTransactions is on, log
any implicit commit of an automatic transaction if write operations
where performed within that transaction.

The idea is to provide warnings about nested explicite transactions
while staying quiet about implicite commits to automatic transactions.

Change-Id: Idbe4a9034b13413e351f432408d358a704f6b77d