Enable some time estimate for --progress option (or a new --progress-total) option.

As `btrbk diff` already delivers a super nice overview of the difference of a parent and child snapshot, one could also get a time estimate during a `btrbk run`. This could be done by extending the existing `--progress` option, or by introducing a `--progress-total`.
Choosing this option, for every newly created snapshot `btrbk diff` would be run before `btrfs send/receive` such that the pv command can get proper `-s` option for the progress update.

This way it is easier to estimate the total time left, especially in cases where the network bandwidth is the limiting factor.

Start with the existing --progress handling in btrbk run and the btrbk diff behavior described in the issue. Investigate how btrfs send/receive invokes pv and how a total size could be supplied for each newly created snapshot. Done means an agreed implementation of --progress or --progress-total that reports a useful total-time estimate during transfers.

Written by the indexing model from the issue text.