Improve output from the rebuild-tracker batch job (drush and UI)

Problem/Motivation

On a site with 1000s (but not millions) of nodes, running drush sapi-rt repeats the following message 100s of times:

  [notice] Successfully executed 0 pending tasks.

Finally, at the very end, it says:

>  [notice] Message: Successfully executed 1 pending task.

If you run the batch via the UI by clicking on "Rebuild tracking information" and submit the confirmation form, you just see :

Successfully executed 0 pending tasks.
Completed 0 of 1.              0%

the whole time, until the very end (many minutes later) when it's finally done with everything.

Steps to reproduce

Either run drush sapi-rt

Or, click on "Rebuild tracking information" link on a SAPI index and click the confirm form and watch the batch process.

Proposed resolution

Either:

A) Don't print something at the end of each batch that we're not done yet, and only print out the single "Successfully executed 1 pending task" at the end if it succeeded, or an error message if not.

B) Print something more relevant at the end of each batch, like "processed 2350 out of 120129 items, tracking 2143" (or whatever).
I'm not yet entirely sure what this batch process is exactly doing, but it seems plausible that we'd need a single count of all entities on the system and then know we're going to investigate 100 at a time to decide if they need to be indexed or not. More or less. So, we could use the batch API to report the total # of batches we're going to need in advance, and then the progress bar would work, and we could actually see something happening in real time.

Remaining tasks

1. Decide an appropriate direction.
2. Implement it.
3. Add/update tests about it.
4. Reviews / refinements.
5. RTBC.
6. Commit.