the size of the UNDO slots cache lists (insert and upate). If either of
cached lists has more than 500 entries then we add any UNDO slots that are
freed, to the common free list instead of the cache list, this is to avoid
the case where all the free slots end up in only one of the lists on startup
after a crash.

Tested with test case for 26590 and passes all mysql-test(s).