[MERGE] Split MemPool.cc into framework and allocators

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 may also fix some statistics bugs. There was some oddness there
with a bit of confusion between allocator local counters and MemMeter
counters..

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

Regards
Henrik