Get rid of status api for logs and buildsteps

The biggest user-visible change here is that the little status strings associated with each step end up changing quite a bit.  Honestly, I couldn't make heads or tails of the five or six ways these strings could be specified (including both `getText` and `setText`, inexplicably).  I replicated most functionality, and just let some stuff slide.  They're just status strings.

See http://trac.buildbot.net/ticket/2818 for details.
(I am also guilty of this kind of changes :))