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

HashTable.cpp

@@ -96,7 +96,9 @@ void ConflictSet::setOldestVersion(int64_t oldestVersion) {
   return impl->setOldestVersion(oldestVersion);
 }
 
-int64_t ConflictSet::getBytes() const { return -1; }
+// The hash_table implementation does not track memory usage, so return 0 to
+// satisfy the API contract that getBytes() returns a non-negative value.
+int64_t ConflictSet::getBytes() const { return 0; }
 
 void ConflictSet::getMetricsV1(MetricsV1 **metrics, int *count) const {
   *metrics = nullptr;
@@ -161,7 +163,10 @@ __attribute__((__visibility__("default"))) void ConflictSet_destroy(void *cs) {
 }
 __attribute__((__visibility__("default"))) int64_t
 ConflictSet_getBytes(void *cs) {
-  using Impl = ConflictSet::Impl;
-  return -1;
+  (void)cs;
+  // The hash_table implementation does not track memory usage, so return 0 to
+  // satisfy the API contract that ConflictSet_getBytes returns a non-negative
+  // value.
+  return 0;
 }
 }