Remove legacy network code (#860)

* expunge legacy code from polkadot-network

* mostly rip out old legacy protocol from service

* ensure validation work is spawned by incoming messages

* decouple availabliity store from network logic; clean up data flow

* av_store: test helpers and use futures-abort

* update polkadot-validation to pass n_validators when submitting chunks

* fallible erasure-chunk fetching

* implement `ErasureNetworking` for new network prot

* API for registering availability store in network

* fully integrate new network service into service

* fix validation tests

* scaffolding for porting collator over to new network

* track connected validators' peer IDs and distribute collators' collations

* helper in network for fetching all checked statements

* fix adder-collator

* actually register notifications protocol

* Update service/src/lib.rs

* merge with master

polkadot/validation/src/pipeline.rs

@@ -137,6 +137,8 @@ pub struct FullOutput {
 	/// should keep their chunk (by index). Other chunks do not need to be
 	/// kept available long-term, but should be distributed to other validators.
 	pub erasure_chunks: Vec<ErasureChunk>,
+	/// The number of validators that were present at this validation.
+	pub n_validators: usize,
 }
 
 impl FullOutput {
@@ -218,6 +220,7 @@ impl<'a> ValidatedCandidate<'a> {
 			available_data,
 			commitments,
 			erasure_chunks: chunks,
+			n_validators,
 		})
 	}
 }