Add round buffer. Closes #3

[juliangruber]

Closes #3.

This PR implements self healing by buffering current round's measurements on disk.

• Write tests
• Write round index to first line of buffer file
• Handle when the recovered round isn't the right one (wrong index)
• Append or create the buffer file (see test failure)

[bajtos]

I am not opposed to this solution, but I'd like to propose an alternative to consider:

• At spark-evaluate startup, make an RPC API call to fetch recent MeasurementsAdded events all the way up to the beginning of the current round. See how dry-run does that:
  https://github.com/filecoin-station/spark-evaluate/blob/0d1cce33cfd137cd185b8c2e9e534ca043e09ceb/bin/dry-run.js#L215-L257

• For each event found, call the preprocess step.

Upsides:

• The service will not need any persistent volume.
• We don't have to write & maintain complex code to manage a ringbuffer.

Downsides:

• When there is an outage of the RPC API, or if the eth_getLogs call takes long time to complete, then spark-evaluat will need a lot of time before it starts & can handle events. (This can be mitigated, though. E.g. the startup does not have to wait until we get all events.)
• On restart, we have to re-fetch measurements from older round events.