Code review comment for lp:~al-maisan/launchpad/ibuilder-api-cleanup-505647

Hello there!

This branch cleans up the IBuilder interface and model code related to
candidate job selection.
The findBuildCandidate()/dispatchBuildCandidate() methods are made internal
and a new findAndStartJob() method that hides them is introduced.

Tests to run:

   bin/test -vv -t build

No "make lint" errors.