(WIP) fix problems with removing an active builder

This my attempt at fixing http://trac.buildbot.net/ticket/2701

When a builder is removed, cancel it's pending build requests and stop it's running builds. It does solves the bug, but I'm not sure it introduces any other problems.

I'm not completely done with this pull request. I still plan to write unit test for the new code, I just want to get some feedback first.