Limit final bundle size in B rather than num of blocks

[segfault-magnet]

Rather than having a limit on a number of blocks that can be part of a bundle, we should limit the total acceptable output size of the bundle. This way we have more consistent throughput in times of high traffic on fuel.

Consider changing what we accumulate to be bytes instead of blocks -- i.e. start the bundling process when we have, e.g. 2MB of block data instead of 3600 blocks or 1h passes since the last bundle.

So, we might start configuring the committer to:

Wait for 4MB of block data or 1h, generate bundles only up to 2MB in size (11-12 blobs).