txn: unify txn_stmt tuples reference counting rules

Currently, the way txn_stmt::old_tuple and new_tuple are referenced
depends on the engine. For vinyl, the rules are straightforward: if
txn_stmt::{old_tuple,new_tuple} is not NULL, then the reference to the
corresponding tuple is elevated. Hence when a transaction is committed
or rolled back, vinyl calls tuple_unref on both txn_stmt::old_tuple and
new_tuple. For memtx, things are different: the engine doesn't
explicitly increment the reference counter of the tuples - it simply
sets them to the newly inserted tuple and the replaced tuple. On commit,
the reference counter of the old tuple is decreased to delete the
replaced tuple, while on rollback the reference counter of the new tuple
is decreased to delete the new tuple.

Because of this, we can't implement the blackhole engine (aka /dev/null)
without implementing commit and rollback engine methods - even though
such an engine doesn't store anything it still has to set the new_tuple
for on_replace trigger and hence it is responsible for releasing it on
commit or rollback. Since commit/rollback are rather inappropriate for
this kind of engine, let's instead unify txn_stmt reference counting
rules and make txn.c unreference the tuples no matter what engine is.
This doesn't change vinyl, because it already conforms. For memtx, this
means that we need to increase the reference counter when we insert a
new tuple into a space - not a big deal as tuple_ref is almost free.