Partial code tidy: move plugin description to end with other items, and added status variable for debug.
Minor fix: make sure alter table wont try to change our storage engine to something else.
Added extra debug to support MDEV-6282
Added regression test for MDEV-6345
Fixed minor spelling mistake.
Minor code cleanup: validation of options to member function.
Hopefully finally fixes MDEV-6282, MDEV-5748 and MDEV-6345