[MERGE] Split MemPool.cc into framework and allocators

Version 2.

Statistics corrected. Never worked proper, and the Malloc pool type did
not get any statistics reported at all due to part of the statistics
being malplaced down in the chunked allocator instead of generic
framework where it belongs.

This change splits MemPool.cc into framework and specific allocators

MemPool.cc general framework
MemPoolChunked.cc chunked allocator
MemPoolMalloc.cc malloc allocator, no freelist (yet)

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.

Next step is adding back the simple freelist to MemPoolMalloc.cc making
it a fullblown pooled allocator, configurable by squid.conf.

Regards
Henrik