atc: notify build tracker when a build starts
atc: notify build tracker when a build starts

This will kick off the build tracker right away when the scheduler
starts a build. The scheduler used to also start the build before, but
this was removed in order to have the build tracker component be the
only thing that starts builds. But this introduced some slowness with
builds being in started state but waiting for a few seconds before steps
start to run.

[#5612]

Signed-off-by: Clara Fu <d5c8bdbc15689555c92563892e959c79b3fc425f@pivotal.io>

concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse,concourse/concourse