Requests submitted first may carry a larger xid resulting in ZRUNTIMEINCONSISTENCY

When multiple threads attempt to submit requests, it's possible for a request from a thread that acquired its xid earlier to be inserted after a request from a thread that acquired its xid later in the submission queue, which causes a ZRUNTIMEINCONSISTENCY error.

To fix it, acquires the lock before get_xid() and releases it after request submission.