MDEV-3982: show_explain.test fails, times out or crashes Backport the fix from 10.0 tree
MDEV-3982: show_explain.test fails, times out or crashes
Backport the fix from 10.0 tree

- The problem was that thd_killed() may be called by innodb from 
  an internal innodb thread.
- Fixed by not processing APC requests when we're not in the thread
  that owns the APC target.


natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,ollie314/server,ollie314/server,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,natsys/mariadb_10.2,natsys/mariadb_10.2,davidl-zend/zenddbi,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,flynn1973/mariadb-aix,davidl-zend/zenddbi,davidl-zend/zenddbi,flynn1973/mariadb-aix,ollie314/server,ollie314/server,ollie314/server,davidl-zend/zenddbi,ollie314/server,ollie314/server,flynn1973/mariadb-aix,flynn1973/mariadb-aix,natsys/mariadb_10.2,davidl-zend/zenddbi,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,natsys/mariadb_10.2,flynn1973/mariadb-aix,ollie314/server,davidl-zend/zenddbi,flynn1973/mariadb-aix,davidl-zend/zenddbi