Extend PipelineProcessWorker to accept a list of builds

See merge request gitlab-org/gitlab-ce!31425

This changes used worker from `BuildProcessWorker`
to `PipelineProcessWorker` to make pipeline
processing much simpler. We process `pipeline_id`,
based on some triggers.

Currently, some of the jobs with `needs:`
would be processed in stages, it means
that `when:` for such jobs would not be
respected.

This changes the behavior to have a separate
execution paths for jobs with `needs:`.

This implements the support for `needs:` keyword
as part of GitLab CI. That makes some of the jobs
to be run out of order.

Prepares us for upgrade to Rails 5.2

Introduces the concept of Prerequisites for a CI build.
If a build has unmet prerequisites it will go through the
:preparing state before being made available to a runner.

There are no actual prerequisites yet, so current
behaviour is unchanged.

Introduces a new status for builds between :created and :pending
that will be used when builds require one or more prerequisite
actions to be completed before being picked up by a runner
(such as creating Kubernetes resources before deploying).

The existing :created > :pending transition is unchanged, so
only builds that require preparation will use the :preparing
status.