journal: add debug mode for mmap-cache (--enable-debug=mmap-cache)

This is useful for exposing unsafe access to mmapped objects after
the context that they were mapped in was already moved.

For example:
journal_file_move_to_object(f1, OBJECT_DATA, p1, &o1);
journal_file_move_to_object(f2, OBJECT_DATA, p2, &o2);
t = o1->object.type; /* this usually works, but is unsafe */