txn: merge struct sql_txn into struct txn

This procedure is processed in several steps. Firstly, we add name
to struct txn_savepoint since we should be capable of operating on named
savepoints (which in turn is SQL feature). Still, anonymous (in the sense
of name absence) savepoints are also valid. Then, we add list (as
implementation of stailq) of savepoints to struct txn: it allows us to
find savepoint by its name. Finally, we patch rollback to/release
savepoint routines: for rollback tail of the list containing savepoints
is cut (but subject of rollback routine remains in the list); for
release routine we cut tail including node being released.