Add a stat for number of BUILD files currently parsing

[peterebden]

We've seen a few cases recently of plz apparently hanging. It's unclear precisely what's going on (the version hasn't changed for some time yet the issues seem recent).

This should help shed a little light on what it's doing. I think it's not a bad stat to see in general.

I decided just to expose the atomic.Int64 object directly, could also wrap it in several functions if needed but I don't think it helps especially. Would be easy enough to change later.

Output looks like this:

09:57:47.482  NOTICE: Build running for 40s, 41405 / 82029 tasks done, 250 workers busy, parsing 22584 BUILD files
09:57:57.481  NOTICE: Build running for 50s, 59727 / 101290 tasks done, 250 workers busy, parsing 19236 BUILD files
09:58:07.481  NOTICE: Build running for 1m0s, 88643 / 117338 tasks done, 250 workers busy, parsing 4390 BUILD files