evidence: retrieve header at height of evidence for validation (#4870)

validation of lunatic evidence requires that the node retrieve the header at the height of the infringement from the block store for comparison

consensus/reactor_test.go

@@ -236,6 +236,7 @@ func (m *mockEvidencePool) IsPending(evidence types.Evidence) bool {
 	return false
 }
 func (m *mockEvidencePool) AddPOLC(types.ProofOfLockChange) error { return nil }
+func (m *mockEvidencePool) Header(int64) *types.Header            { return nil }
 
 //------------------------------------