Clarify field names for simplex p2p messages (#4002)

Author: samliok

proto/p2p/p2p.proto

@@ -473,22 +473,22 @@ message Signature {
 }
 
 message Vote {
-  BlockHeader vote = 1;
+  BlockHeader block_header = 1;
   Signature signature = 2;
 }
 
 message EmptyVote {
-  ProtocolMetadata vote = 1;
+  ProtocolMetadata metadata = 1;
   Signature signature = 2;
 }
 
 message QuorumCertificate {
-  BlockHeader finalization = 1;
+  BlockHeader block_header = 1;
   bytes quorum_certificate = 2;
 }
 
 message EmptyNotarization {
-  ProtocolMetadata empty_vote = 1;
+  ProtocolMetadata metadata = 1;
   bytes quorum_certificate = 2;
 }