Fix a regression memory leak introduced by refactoring.

Fix a memory leak in Lua stored procedures introduced
when cherry-picking changes from the old net-split
tree.

Make a palloc savepoint *before* we allocate txn
and port, to make sure txn and port memory is freed
when rolling back to the palloc savepoint.
Otherwise a long-running Lua procedure (e.g. background
fiber) can keep allocating palloc memory indefinitely.