rewrite network code to use notifications_protocol APIs from Substrate (#788)

* extract all network code to legacy submodule

* update references to legacy proto

* skeleton of futures-based protocol

* refactor skeleton to use background task

* rename communication_for to build_table_router

* implement internal message types for validation network

* basic ParachainNetwork and TableRouter implementations

* add some module docs

* remove exit-future from validation

* hack: adapt legacy protocol to lack of exit-future

* generalize RegisteredMessageValidator somewhat

* instantiate and teardown table routers

* clean up RouterInner drop logic

* implement most of the statement import loop

* implement statement loop in async/await

* remove unneeded TODO

* most of the collation skeleton

* send session keys and validator roles

* also send role after status

* use config in startup

* point TODO to issue

* fix test compilation

polkadot/validation/src/shared_table/mod.rs

@@ -620,6 +620,7 @@ mod tests {
 	struct DummyRouter;
 	impl TableRouter for DummyRouter {
 		type Error = ::std::io::Error;
+		type SendLocalCollation = future::Ready<Result<(),Self::Error>>;
 		type FetchValidationProof = future::Ready<Result<PoVBlock,Self::Error>>;
 
 		fn local_collation(
@@ -628,7 +629,7 @@ mod tests {
 			_candidate: CandidateReceipt,
 			_outgoing: OutgoingMessages,
 			_chunks: (ValidatorIndex, &[ErasureChunk])
-		) {}
+		) -> Self::SendLocalCollation { future::ready(Ok(())) }
 
 		fn fetch_pov_block(&self, _candidate: &CandidateReceipt) -> Self::FetchValidationProof {
 			future::ok(pov_block_with_data(vec![1, 2, 3, 4, 5]))