Clean up lockorder data of destroyed mutexes

The lockorder potential deadlock detection works by remembering for each
lock A that is acquired while holding another B the pair (A,B), and
triggering a warning when (B,A) already exists in the table.

A and B in the above text are represented by pointers to the CCriticalSection
object that is acquired. This does mean however that we need to clean up the
table entries that refer to any critical section which is destroyed, as it
memory address can potentially be used for another unrelated lock in the future.

Implement this clean up by remembering not only the pairs in forward direction,
but also backward direction. This allows for fast iteration over all pairs that
use a deleted CCriticalSection in either the first or the second position.