raft_serverpb: add new ExtraMessage types for the snapshot gen preche…

…ck process

This commit adds two new new ExtraMessage types that are part of the snapshot
gen precheck process.

Before a leader generates a snapshot, it sends a precheck request to the target
follower. The follower responds with a boolean value indicating whether the
check succeeds.

Signed-off-by: Bisheng Huang <[email protected]>

proto/raft_serverpb.proto

@@ -252,9 +252,9 @@ enum ExtraMessageType {
     MsgGcPeerResponse = 12;
     MsgFlushMemtable = 13;
     MsgRefreshBuckets = 14;
-
-    MsgSnapshotSendPrecheckRequest = 15;
-    MsgSnapshotSendPrecheckResponse = 16;
+    // Messages for the snapshot gen precheck process.
+    MsgSnapGenPrecheckRequest = 15;
+    MsgSnapGenPrecheckResponse = 16;
 }
 
 message FlushMemtable {
@@ -296,7 +296,7 @@ message ExtraMessage {
     AvailabilityContext availability_context = 8;
     // notice the peer to refresh buckets version
     RefreshBuckets refresh_buckets = 9;
-    // A snapshot is only sent by the leader after this precheck, conducted on 
-    // the follower, has successfully passed. 
-    bool snapshot_precheck_passed = 10;
+    // snapshot_gen_precheck_passed is used to indicate the precheck result when
+    // a follower responds to a leader's precheck request.
+    bool snapshot_gen_precheck_passed = 10;
 }