Return 0 instead of -1 from hash_table getBytes()

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

test_conflict_set.py

@@ -57,6 +57,17 @@ def test_conflict_set():
         assert cs.check(read(0, key), read(1, key)) == [Result.TOO_OLD, Result.COMMIT]
 
 
+def test_hash_table_getBytes():
+    # Regression test for issue #62: the hash_table implementation is
+    # point-query only and does not track memory usage, but getBytes() must
+    # still return a non-negative value rather than -1.
+    with ConflictSet(0, build_dir=build_dir, implementation="hash_table") as cs:
+        assert cs.getBytes() == 0
+        cs.addWrites(1, write(b"key"))
+        assert cs.getBytes() >= 0
+        assert cs.check(read(0, b"key")) == [Result.CONFLICT]
+
+
 def test_write_read_without_outer_reference():
     # Regression test for issue #42: WriteRange/ReadRange must keep their
     # backing key buffers alive, because the C library reads the pointer