hold the latch across the 2-phase commit, and it is already released at
start of prepare (if not before).

Rename trx->active_trans to active_flag; since we changed the semantics of
the field, renaming should help prevent silent merge errors.

Tweak the commit_ordered() semantics. Now it is only called for transactions
that go through 2-phase commit. This avoids forcing engines to make commits
visible before they are durable.

Also take LOCK_commit_ordered() around START TRANSACTION WITH CONSISTENT
SNAPSHOT, to get a truly consistent snapshot.