Light GRANDPA import handler (#1669)

* GrandpaLightBlockImport

* extract authorities in AuraVerifier

* post-merge fix

* restore authorities cache

* license

* new finality proof draft

* generalized PendingJustifications

* finality proof messages

* fixed compilation

* pass verifier to import_finality_proof

* do not fetch remote proof from light import directly

* FinalityProofProvider

* fixed authorities cache test

* restored finality proof tests

* finality_proof docs

* use DB backend in test client

* justification_is_fetched_by_light_client_when_consensus_data_changes

* restore justification_is_fetched_by_light_client_when_consensus_data_changes

* some more tests

* added authorities-related TODO

* removed unneeded clear_finality_proof_requests field

* truncated some long lines

* more granular light import tests

* only provide finality proof if it is generated by the requested set

* post-merge fix

* finality_proof_is_none_if_first_justification_is_generated_by_unknown_set

* make light+grandpa test rely on finality proofs (instead of simple justifications)

* empty_finality_proof_is_returned_to_light_client_when_authority_set_is_different

* missing trait method impl

* fixed proof-of-finality docs

* one more doc fix

* fix docs

* initialize authorities cache (post-merge fix)

* fixed cache initialization (post-merge fix)

* post-fix merge: fix light + GRANDPA tests (bad way)

* proper fix of empty_finality_proof_is_returned_to_light_client_when_authority_set_is_different

* fixed easy grumbles

* import finality proofs in BlockImportWorker thread

* allow import of finality proofs for non-requested blocks

* limit number of fragments in finality proof

* GRANDPA post-merge fix

* BABE: pos-merge fix

substrate/core/network/src/lib.rs

@@ -30,6 +30,7 @@ mod protocol;
 mod chain;
 mod blocks;
 mod on_demand;
+mod extra_requests;
 mod util;
 pub mod config;
 pub mod consensus_gossip;
@@ -40,7 +41,7 @@ pub mod specialization;
 #[cfg(any(test, feature = "test-helpers"))]
 pub mod test;
 
-pub use chain::Client as ClientHandle;
+pub use chain::{Client as ClientHandle, FinalityProofProvider};
 pub use service::{
 	Service, FetchFuture, TransactionPool, ManageNetwork, NetworkMsg,
 	SyncProvider, ExHashT, ReportHandle,