option to use length instead of timestamp

@dominictarr: If you used length-so-far as the ts then this would be great for storing files,
and replicating them, because, if you made the buffers all block-size then they would end up the same in each database, with the same names.

The issue names no files or tests. Start by tracing where timestamps are assigned and how buffers are named across LevelDB stores; done means an opt-in length-based mode is specified and verified for storing and replicating files.

Written by the indexing model from the issue text.