Ensure that table router is always built (#952)

* Ensure that table router is always build

This pr ensures that the table router is always build, aka the future is
resolved. This is important, as the table router internally spawns tasks
to handle gossip messages. Handling gossip messages is not only required
on parachain validators, but also on relay chain validators to receive collations.

Tests are added to ensure that the assumptions hold.

* Fix compilation

* Switch to closures

* Remove empty line

* Revert "Remove empty line"

This reverts commit a6c19438.

* Revert "Switch to closures"

This reverts commit b989c303.

* Hybrid approach

* Rename test

* Make trait crate local