evidence: create proof of lock change and implement it in evidence store (#4746)

creates a proof of lock change which is an array of votes that proves the validator was permitted to change locked block and vote again. This proof is stored in the evidence pool and is used as a part of amnesia evidence

consensus/reactor_test.go

@@ -235,6 +235,7 @@ func (m *mockEvidencePool) IsPending(evidence types.Evidence) bool {
 	}
 	return false
 }
+func (m *mockEvidencePool) AddPOLC(types.ProofOfLockChange) error { return nil }
 
 //------------------------------------