txn: destroy commit and rollback triggers

They were not deleted ever. Worked fine for DDL and replication,
for which they were introduced in the first place, because these
triggers are on the region memory.

But didn't work when the triggers became available in the public
API, because these are allocated on the heap. As a result, all the
box.on_commit() and box.on_rollback() triggers leaked.

The patch ensures all the on_commit/on_rollback triggers are
destroyed.

The statement triggers on_commit/on_rollback are left intact since
they are private and never need deletion, but the patch adds
assertions in case they ever would need to be destroyed.

Another option was to force all the commit and rollback triggers
clear themselves. For example, in case of commit all the on_commit
triggers must clear themselves, and the rollback triggers are
destroyed. Vice versa when a rollback happens. This would allow
not to manually destroy on_commit triggers in case of commit. But
it didn't work because the Lua triggers all work via a common
runner lbox_trigger_run(), which can't destroy its argument in
most of the cases (space:on_replace, :before_replace, ...). It
could be patched but requires to add some work to the Lua triggers
affecting all of them, which in total might be not better.

Closes #6025