Validation service refactoring (#773)

* add some more docs about statement import

* instantiate environment async

* move attestation service into subfolder

* refactor validation service architecture somewhat

* remove dependence on validation service in proposer

* fix a bunch of warnings

* improve docs

* introduce a builder for the validation service

* extract block production to its own file

* integrate new API into service

* address review grumbles

polkadot/validation/src/error.rs

@@ -46,6 +46,8 @@ pub enum Error {
 	Timer(std::io::Error),
 	#[display(fmt = "Failed to compute deadline of now + {:?}", _0)]
 	DeadlineComputeFailure(std::time::Duration),
+	#[display(fmt = "Validation service is down.")]
+	ValidationServiceDown,
 	Join(tokio::task::JoinError)
 }