Enable dedicated streams to start from interruption

[morgsmccauley]

After #418, each Indexer will have it's own dedicated stream. To persist these streams across restarts, we need to store the last_indexed_block in Redis, so that it can continue where it left off, essentially "Start from interruption".

This will need to be implemented in Coordinator V1, so when the new Control Plane starts, it's able to continue where it left off. But, this should also be implemented in #418, within the new Block Streamer service.