Keep the table router for the lifetime of the validation instance alive (#1175)

This pr ensures that the table router stays alive for the lifetime of
the validation instance. This is required to ensure that the node
responds onto gossip messages for the particular relay chain round.
Before, the table router was only kept alive for relay chain nodes that
were assigned to a Parachain, however the lifetime was also relative
short. This lead to bugs where a relay chain node did not include
PoVBlock's, because it did not receive them (rejected them on receive,
because it was not listening on the particular round).

polkadot/validation/src/error.rs

@@ -47,6 +47,9 @@ pub enum Error {
 	/// Proposer destroyed before finishing proposing or evaluating
 	#[display(fmt = "Proposer destroyed before finishing proposing or evaluating")]
 	PrematureDestruction,
+	/// Failed to build the table router.
+	#[display(fmt = "Failed to build the table router: {}", _0)]
+	CouldNotBuildTableRouter(String),
 	/// Timer failed
 	#[display(fmt = "Timer failed: {}", _0)]
 	Timer(std::io::Error),