Keep track of each recently active XID, recording which worker it was queued
on. If an XID might still be active, choose the same worker to queue event
groups that refer to the same XID to avoid conflicts.

Otherwise, schedule the XID freely in the next round-robin slot.

This way, XA PREPARE can normally be scheduled without restrictions (unless
duplicate XID transactions come close together). This improves scheduling
and parallelism over the old method, where the worker thread to schedule XA
PREPARE on was fixed based on a hash value of the XID.

XA COMMIT will normally be scheduled on the same worker as XA PREPARE, but
can be a different one if the XA PREPARE is far back in the event history.

Testcase and code for trimming dynamic array due to Andrei.
Reviewed-by: Andrei Elkin <andrei.elkin@mariadb.com>
Signed-off-by: Kristian Nielsen <knielsen@knielsen-hq.org>