txn: reverse commit trigger list only before running commit triggers

Commit triggers must be run in the same order they are added, see commit
01343264 ("txn: fix execution order of commit triggers"). To achieve
that we added a new trigger method, trigger_add_tail(), which adds new
triggers to the trigger list tail rather than to the head, and now we
use this new method for adding commit triggers.

Come to think of it now, that solution was rather confusing. First,
commit triggers are still added to the head of the list from Lua.
Second, to revert triggers on rollback-to-savepoint it would be really
more convenient to have both commit and rollback trigger lists have the
same order.

So this patch reverts the above-mentioned commit and instead simply
uses a reverse iterator to run commit triggers.