[MERGE] Split MemPool.cc into framework and allocators

Version 4. Blank out memory chunk statistics when not using chunked
allocator and update configure.in to match.

This change splits MemPool.cc into framework and specific allocators

MemPool.cc general framework
MemPoolChunked.cc chunked allocator
MemPoolMalloc.cc malloc based allocator with freelist

This also fixes some statistics bugs. There was some bad oddness there
with a bit of confusion between allocator local counters and MemMeter
counters, plus total confusion between allocation and saved allocation.

Regards
Henrik