[MERGE] Split MemPool.cc into framework and allocators

Version 3. Adds pooling by use of a free list to the simple malloc based
pool type.

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