Close all RocksIterators

[fm3]

The caller should close the Iterator returned by RocksDB.newIterator. That’s us.

I ran this stress test

    for i in range(100000):
        reply = stub.ListVersions(proto.ListVersionsRequest(collection=collection, key=volume_id))
        assert_success(reply)

and on master, fossilDB memory usage went up by about 1.5 GB for each run. On this branch, it stayed steady even after multiple runs. Speed seems unaffected (variations are smaller than noise)

[normanrz]

Good stuff!