Merge block watcher and committer

[segfault-magnet]

The buffering between the block watcher and committer turned out to be unneeded in hindsight. Merge the two components into one that will:

• check for new blocks
• should they be committed (not committed, epoch block)
• have we committed all the data needed for that blob?
• if so then proceed to commit, otherwise wait.

Also investigate the possibility of not having to commit every EPOCH blocks in order to simplify failure handling logic.