From the correctness point of view, it should be safe to release
all locks on index records that were not modified by the transaction.
Doing so should make the locks after XA PREPARE fully compatible
with what would happen if the server were restarted: InnoDB table
IX locks and exclusive record locks would be resurrected based on
undo log records.

Concurrently running transactions that are waiting for a lock may invoke
lock_rec_convert_impl_to_expl() to create an explicit record lock object
on behalf of the lock-owning transaction so that they can attaching
their waiting lock request on the explicit record lock object. Explicit
locks would be released by trx_t::release_locks() during commit or
rollback.

Any clustered index record whose DB_TRX_ID belongs to a transaction that
is in active or XA PREPARE state will be implicitly locked by that
transaction. On XA PREPARE, we can release explicit exclusive locks on
records whose DB_TRX_ID does not match the current transaction identifier.

lock_rec_unlock_unmodified(): Release record locks that are not implicitly
held by the current transaction.

lock_release_on_prepare_try(), lock_release_on_prepare():
Invoke lock_rec_unlock_unmodified().

row_trx_id_offset(): Declare non-static.

lock_rec_unlock(): Replaces lock_rec_unlock_supremum().

Reviewed by: Vladislav Lesin