grandpa: pluggable voting rules (#3673)

* grandpa: support pluggable custom voting rules

* grandpa: add docs to grandpa voting rule

* grandpa: make voting rule mandatory

* grandpa: add test for voting rule

* node: add GRANDPA voting rule

* grandpa: pass backend to VotingRule

* core: fix docs in SelectChain::finality_target

* grandpa: implement 3/4 of unfinalized chain restriction as voting rule

* grandpa: rename AlwaysBehindBestBlock voting rule

* grandpa: fix tests

* grandpa: remove useless test

* grandpa: extend environemnt voting rule test

* grandpa: add proofs to unreachable statements

* grandpa: fix typo

* grandpa: fix docs

substrate/node/cli/src/service.rs

@@ -205,6 +205,7 @@ macro_rules! new_full {
 					inherent_data_providers: inherent_data_providers.clone(),
 					on_exit: service.on_exit(),
 					telemetry_on_connect: Some(service.telemetry_on_connect_stream()),
+					voting_rule: grandpa::VotingRulesBuilder::default().build(),
 				};
 				service.spawn_task(Box::new(grandpa::run_grandpa_voter(grandpa_config)?));
 			},