bg worker: patch 6 of 6 - ooo messages

Finally, this patch adds the capability to cache out-of-order messages
for workers within the coordinator process. Unlike the cache of jobs,
which basically are messages as well, these messages don't trigger a
job, but might provide additional information or data payload to a
worker that's already processing a job.

In case of Postgres-R, the very first change set received starts an
applicator job, while every subsequent change set as well as the final
commit decision are such ooo-messages, which are forwarded to the same
worker.

A parallel querying feature might want to use similar ooo messages for
data payload, forth and back. However, simpler jobs like vacuum don't
necessarily need this.