HEURISTICS FOR COMPRESSED PAGE SIZE

The fix of Bug#12845774 was supposed to skip known-to-fail
btr_cur_optimistic_insert() calls. There was only one such call, in
btr_cur_pessimistic_update(). All other callers of
btr_cur_pessimistic_insert() would release and reacquire the B-tree
page latch before attempting the pessimistic insert. This would allow
other threads to restructure the B-tree, allowing (and requiring) the
insert to succeed as an optimistic (single-page) operation.

Failure to attempt an optimistic insert before a pessimistic one would
trigger an attempt to split an empty page.

rb:1234 approved by Sunny Bains