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