innodb_debug_sync was introduced in commit
b393e2cb and reverted in
commit fc58c172 due to memory leak reported
by valgrind, see MDEV-21336.

The leak is now fixed by adding `rw_lock_free(&slot->debug_sync_lock)`
after background thread working loop is finished, and the patch is
reapplied, with respect to c++98 fixes by Marko.

The missing DEBUG_SYNC for MDEV-18546 in row0vers.cc is also reapplied.