* Remove old service, 3rd try

i.e.
Revert "Revert "Remove Old Service, 2nd try (#1732)" (#1758)"

This reverts commit c80f7b6b.

Closes #1757.

We now have some evidence that the polkadot validator was producing
blocks after all; the reason the blocks_constructed metric was 0 was
that as a new metric it hadn't yet been incorporated into that
branch's codebase. See
https://github.com/paritytech/polkadot/issues/1757#issuecomment-700977602



As this PR is based on a newer `master` branch than the previous one,
that should hopefully no longer be an issue.

* paras trait now has an Origin type

* initial work running a two node local net

* use the right incantations so the nodes produce blocks together

* improve internal documentation

Co-authored-by: Bastian Köcher <[email protected]>