Do not retry proposal if already found in CommittedEntries (#2740)

This is the first win from Open Census. Learnt that proposals immediately show up in `raft.Ready().CommittedEntries`, but then take a long time in the `applyCh` queue to be applied. If these proposals are just waiting in the queue to be applied, then we're sure that Raft didn't lose them in flight, and therefore, we don't need to re-propose them. This PR detects that and avoids retrying the proposal.

conn/raft_server.go

@@ -52,8 +52,9 @@ func (r *lockedSource) Seed(seed int64) {
 }
 
 type ProposalCtx struct {
-	Ch  chan error
-	Ctx context.Context
+	Found uint32
+	Ch    chan error
+	Ctx   context.Context
 }
 
 type proposals struct {

protos/pb.proto

@@ -211,13 +211,14 @@ message Snapshot {
 }
 
 message Proposal {
-	Mutations mutations                   = 2;
-	repeated KV kv                        = 4;
-	MembershipState state                 = 5;
-	string clean_predicate                = 6; // Delete the predicate which was moved to other group.
-	string key                            = 7;
-	OracleDelta delta                     = 8;
-	Snapshot snapshot                     = 9; // Used to tell the group when to take snapshot.
+	Mutations mutations    = 2;
+	repeated KV kv         = 4;
+	MembershipState state  = 5;
+	string clean_predicate = 6; // Delete the predicate which was moved to other group.
+	string key             = 7;
+	OracleDelta delta      = 8;
+	Snapshot snapshot      = 9; // Used to tell the group when to take snapshot.
+	uint64 index           = 10; // Used to store Raft index, in raft.Ready.
 }
 
 message KVS {