Change MetadataCache to just use malloc/free.
IIRC we never had any evidence that the performance impact of a
separate allocator here was actually measurable, and it does come
at a significant fragmentation cost because every single cache
allocates at least a page of memory. Sharing that with the system
allocator makes more sense, even if these allocations are typically
permanent.
This also means that standard memory-debugging tools will actually
find problems with out-of-bounds accesses to metadata.
