Allow enqueuer to enqueue multiple jobs with the same instance

[johha]

Refactor enqueuer so that it can enqueue multiple jobs with the same instance.
This will allow in the future to e.g. pass down an enqueuer instance to ensure consistent priorities.

Instead of providing a job in initialize it will be provided in enqueue(job), enqueue_pollable(job), or run_inline(job):

class Enqueuer
      def initialize(opts={})
        @opts = opts
        @timeout_calculator = JobTimeoutCalculator.new(VCAP::CloudController::Config.config)
        @priority_overwriter = JobPriorityOverwriter.new(VCAP::CloudController::Config.config)
        load_delayed_job_plugins
      end

      def enqueue(job)
        enqueue_job(job)
      end

      def enqueue_pollable(job, existing_guid: nil)
        wrapped_job = PollableJobWrapper.new(job, existing_guid:)

        wrapped_job = yield wrapped_job if block_given?

        delayed_job = enqueue_job(wrapped_job)
        PollableJobModel.find_by_delayed_job(delayed_job)
      end

      def run_inline(job)
        run_immediately do
          Delayed::Job.enqueue(TimeoutJob.new(job, job_timeout(job)), @opts)
        end
      end

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[kathap]

Needs this line in service_instances_spec.rb to be changed to the new design?
allow_any_instance_of(VCAP::CloudController::Jobs::Enqueuer).to receive(:enqueue_pollable).and_raise(Sequel::DatabaseDisconnectError) no, no need for it :)