erebus: feature completeness

[alexanderbez]

There are a few core outstanding items needed for the erebus tool to be considered complete:

1. Implement a consumer, e.g. Google Bigtable. Modify the existing Consumer interface as necessary.
2. Design and implement retroactive state stream proxying. As it stands today, erebus will watch for new files created from the SDK's state streaming file consumer. If the erebus tool is stopped or if a fail fails to be proxied to another consumer for whatever reason, it will miss one or more state streamed files. So it is necessary for any supported consumer to be able to respond to queries like, "what is the latest state streamed height?". Using this info, erebus will be able to scan back to files that were missed.