Use the new index key iterator for dependency check hashes

In normal transactions this is but a drop in the ocean. However on
rpm -Va, the hashes get rebuilt from scratch on every single package
which on my moderate rpmdb (~2800 packages) testcase results in
103418347 data values fetched and added to dbi sets that are only
thrown away.

With bdb and lmdb this is only a minor optimization but for ndb and sqlite
which can retrieve keys independently, this is a much bigger win. In case
of sqlite, it's a massive one.

Update all backends to allow for NULL set in the index retrieve to
signal key-only retrieval.

lib/depends.c

@@ -938,7 +938,7 @@ static void addIndexToDepHashes(rpmts ts, rpmDbiTag tag,
 {
     char *key;
     size_t keylen;
-    rpmdbIndexIterator ii = rpmdbIndexIteratorInit(rpmtsGetRdb(ts), tag);
+    rpmdbIndexIterator ii = rpmdbIndexKeyIteratorInit(rpmtsGetRdb(ts), tag);
 
     if (!ii)
 	return;