The function trans_rollback_to_savepoint(), unlike trans_savepoint(),
did not allow xa_state=XA_ACTIVE, so an attempt to do ROLLBCK TO SAVEPOINT
inside an XA transaction incorrectly returned an error
"...command cannot be executed ... in the ACTIVE state...".

Partially merging a MySQL patch:
  7fb5c47390311d9b1b5367f97cb8fedd4102dd05
  This is WL#7193 (Decouple THD and st_transactions)...

The currently merged part includes these changes:
- Introducing st_xid_state::check_has_uncommitted_xa()
- Reusing it in both trans_rollback_to_savepoint() and trans_savepoint(),
  so now both allow XA_ACTIVE.