SourceLayout: shuffle memory pool allocators to mem/libmem.la

SourceLayout: shuffle memory pool allocators to mem/libmem.la

There are no logic changes in this patch.

* Some code is using the 'old' or internal pool allocation APIs
  directly. These have been marked with XXX for future logic
  changes to update.

* The memory pools API and MEMPROXY_CLASS() definition are now
  available through the mem/forward.h header file.

* Mem:: namespace has been added for library contents.

* MemAllocatorProxy class has been moved into Mem:: namespace
  and extracted from mem.cc but should only be used via the
  MEMPROXY_CLASS() macro.

* Extended the memory STUB file to define all mem/forward.h
  symbols and a few others needed currently by code using the
  older deprecated memory APIs.

Also, some time API functions exported by util.h instead of
SquidTime.h have been marked as needing work.