Track memory consumption in a Counter.

Anticipating the elimination of MemoryPool, duplicate tracking of memory
consumption in a simple "Counter" object.
Make SFWriterElem an ordinary object.

Stop allocating from MemoryPool.
Use an ordinary Hash for uniquing values.

Stop using a ZombieKeyedHash, with values allocated from a MemoryPool,
and instead use ordinary String, etc.
Eliminate MemoryPool from SortFieldWriter.

SortFieldWriter used to allocate values from the MemoryPool, which was
also used to track memory usage.  Now it uses ordinary objects and
memory consumption is tracked in a simple Counter, so the MemoryPool is
no longer needed.
Remove now-unused ZombieKeyedHash.