Code review comment for lp:~parthm/bzr/538868-message-for-heavy-checkout

With a lot of help from John, this patch is is a good enough shape for review.
Its evolved from a fix for bug #538868 to a fix for bug #374740

The intent is to show users an _estimate_ of the amount of work pending in branch/push/pull/checkout (remote-local, local-remote, remote-remote) operations. This is done by showing the number of records pending.

E.g.

[tmp]% ~/src/bzr.dev/edge/bzr checkout ~/src/bzr.dev/trunk pqr
- Fetching revisions:Inserting stream:Estimate 106429/320381

As the number of records are proportional to the number of revisions to be fetched, for remote operations, this count is not known and the progress bar starts with "Estimating.. X" where X goes from 0 to revs-to-fetch, following this the progress bar changes to whats shown above. For the local ops, we know the count upfront so the progress starts at 0/N.

A RecordCounter object has been added to maintain current, max, key_count and to encapsulate the estimation algorithm. An instance of this is added to StreamSource which is then used among the various sub-streams to show progress. The wrap_and_count generator wraps existing sub-streams with the progress bar printer.