Rework memory pools

Rework memory pools

- Default to the old simple non-chunked pool type using malloc
  with a simple freelist per pool.
- Various statistics & cachemgr Memory Utilization fixes
- Source reorganisation to split pool implementations from the general
  framework. Allocators now in MemPoolChunked.cc and MemPoolMalloc.cc,
  with general framework & statistics in MemPool.cc.

The chunked allocator is still available and can be activated by setting
the environment variable MEMPOOLS=1 but the default is now the non-chunked
allocator which has been fixed to account allocations properly for statistics
and also maintain a simple freelist to cache allocations.

No squid.conf changes, other than that the memory_pools and
memory_pools_limit directives now works as intended (not possible
with the chunked allocator).