The hash_table implementation cannot accurately track memory usage and was
returning -1 from ConflictSet::getBytes() and ConflictSet_getBytes(). That
violates the API contract that getBytes() returns a non-negative byte count.
Change both entry points to return 0 and document in ConflictSet.h that
implementations which do not track memory usage may return 0.
Add a regression test in test_conflict_set.py that loads the hash_table
implementation and verifies getBytes() is non-negative.
Closes#62
weaselbot