frame/beefy: add privileged call to reset BEEFY consensus (#1534)

We want to be able to (re)set BEEFY genesis in order to (re)start BEEFY
consensus on chains which didn't run it since genesis.

This commit adds privileged helper call to (re)set BEEFY genesis to some
block in the future.

Signed-off-by: Adrian Catangiu <adrian@parity.io>

substrate/frame/beefy/src/default_weights.rs

@@ -49,4 +49,8 @@ impl crate::WeightInfo for () {
 			// fetching set id -> session index mappings
 			.saturating_add(DbWeight::get().reads(2))
 	}
+
+	fn set_new_genesis() -> Weight {
+		DbWeight::get().writes(1)
+	}
 }