box: factor fiber_gc out of txn_commit

Now txn_commit is judge, jury and executioner. It both
commits and rollbacks data, and collects it calling fiber_gc,
which destroys the region.

But SQL wants to use some transactional data after commit. It is
autogenerated identifiers - a list of sequence values generated
for autoincrement columns and explicit sequence:next() calls.

It is possible to store the list on malloced mem inside Vdbe, but
it complicates deallocation. Much more convenient to store all
transactional data on the transaction memory region, so it would
be freed together with fiber_gc.

After this patch applied, Vdbe takes care of txn memory
deallocation in a finalizer routine. Between commit and
finalization transactional data can be serialized wherever.

Needed for #2618