[mc] Bring back KSM (MADV_MERGEABLE) support

[mc] Bring back KSM (MADV_MERGEABLE) support

For KSM support, we allocate the memory for the snapshot regions
directly with mmap() instead of using malloc(). This way we can safely
mark them as MERGEABLE without impacting the heap at all.

Another solution would be to use posix_memalign() (now that we are
using a full-featured malloc) to allocate page-aligned buffers and
remove MADV_UNMERGEABLE when deallocating.