Backport these commits
commit 4ef437558ae8a278f24726276bec57141db086da
Author: Monty <monty@mariadb.org>
Date: Mon Jan 13 18:30:13 2020 +0200
Improve update handler (long unique keys on blobs)
commit 0515577d128318e1b62511846d88d0c56226168d
Author: Sergei Golubchik <serg@mariadb.org>
Date: Thu Mar 5 19:19:57 2020 +0100
cleanup: prepare "update_handler" for WITHOUT OVERLAPS
commit f3f31eaa8e8a07adc0d6769f77f1ccfb129529de
Author: Sergei Golubchik <serg@mariadb.org>
Date: Fri Jan 24 21:10:07 2020 +0100
bugfix: in long uniques don't check for duplicates more than once