Disable the internal memory allocator by default

Disable the internal memory allocator by default

For small sized allocations, the internal allocator gets the memory in
bigger blobs that gets splits into right-sized chunks.  This increases
speed of small allocations and reduced the fragmentation, but such
memory is never released back to the operating system.

Disable the internal allocator by default, and add new `-M internal`
command line option to `named`.