Multiple execution pipelines

[msarahan]

Each new commit is a new pipeline. Right now, we're overwriting old execution pipelines. This is wrong, and probably going to cause problems.

Each execution pipeline needs to be tied to a particular PR, commit, or range of commits.

We also then need a housekeeping policy to keep the number of pipelines at a manageable number. This is critical due to Concourse's current lack of webhooks (concourse/concourse#331), meaning that all pipelines have some polling going on.

[msarahan]

This is fixed. Each different git commit to the monitored repo is a different pipeline. We still need to figure out the housekeeping.