RocksDB ReadOptions memory leak

Currently our rocksdb store impl creates a new ReadOptions instance per iterator created. As per the RocksDB we should be calling dispose() on this once it goes out of scope, so that the c++ underlying impl can free the memory. Currently we're not doing this, resulting in a memory leak.

As we only ever use the same ReadOptions for all iterators, we should simply re-use a singleton for all iterators and then dispose of this on shutdown.