Out-of-band last resort block publication mechanism

[paulperegud]

This is a dead simple / last resort / out-of-band, competence-separated addition to #1277.

1. Create an S3 bucket for block storage, with contents readable by the world, writable by the ch-ch.
2. Let ch-ch put new every block into that storage as a part of "I write block to storage synchronously before committing to the block on Ethereum".
3. Let watchers check S3 bucket if retrieving block from the child chain fails.
4. Seal management access credentials for the account that owns S3 bucket into an envelope.
5. Keep copies of the envelope in the a few safe locations.