varnishhist varient: show TxStatus in histogram form?

[Jason Price]

Doing a bunch of load testing with new code.  I'm trying to get
visibility into http codes.  Most of the time Varnish isn't
responsible for the errors, but it'd be nice to have visibility into:

||
||              +
||      +      +
||      +      +
===========================
200  503v 503b .....

where 503v indicates varnish sent the 503 due to backend timeout, or
503b where the backend delivered the 503 to varnish.

At high load, efficiently parsing varnishlog output is rather a
pain... it'd be nice if varnish did it itself....