make ChangeSet and KVPair protobuf serializable

Implement protobuf messages, and setup buf.build

```
$ buf generate proto
```

CHANGELOG.md

@@ -5,6 +5,7 @@
 ### Improvements
 
 - [#703](https://github.com/cosmos/iavl/pull/703) New APIs `NewCompressExporter`/`NewCompressImporter` to support more compact snapshot format.
+- [#726](https://github.com/cosmos/iavl/pull/726) Make `KVPair` and `ChangeSet` serializable with protobuf.
 
 ### Breaking Changes
 

buf.gen.yaml

@@ -0,0 +1,11 @@
+version: v1
+managed:
+  enabled: true
+  go_package_prefix:
+    default: github.com/cosmos/iavl/gen
+    except:
+      - buf.build/cosmos/gogo-proto
+plugins:
+  - plugin: buf.build/protocolbuffers/go
+    out: gen
+    opt: paths=source_relative

diff.go

@@ -2,18 +2,14 @@ package iavl
 
 import (
 	"bytes"
-)
 
-// ChangeSet represents the state changes extracted from diffing iavl versions.
-type ChangeSet struct {
-	Pairs []KVPair
-}
+	v1 "github.com/cosmos/iavl/gen/v1"
+)
 
-type KVPair struct {
-	Delete bool
-	Key    []byte
-	Value  []byte
-}
+type (
+	KVPair    = v1.KVPair
+	ChangeSet = v1.ChangeSet
+)
 
 // KVPairReceiver is callback parameter of method `extractStateChanges` to receive stream of `KVPair`s.
 type KVPairReceiver func(pair *KVPair) error

diff_test.go

@@ -42,14 +42,14 @@ func genChangeSets(r *rand.Rand, n int) []ChangeSet {
 	var changeSets []ChangeSet
 
 	for i := 0; i < n; i++ {
-		items := make(map[string]KVPair)
+		items := make(map[string]*KVPair)
 		start, count, step := r.Int63n(1000), r.Int63n(1000), r.Int63n(10)
 		for i := start; i < start+count*step; i += step {
 			value := make([]byte, 8)
 			binary.LittleEndian.PutUint64(value, uint64(i))
 
 			key := fmt.Sprintf("test-%d", i)
-			items[key] = KVPair{
+			items[key] = &KVPair{
 				Key:   []byte(key),
 				Value: value,
 			}
@@ -65,7 +65,7 @@ func genChangeSets(r *rand.Rand, n int) []ChangeSet {
 				if pair.Delete {
 					continue
 				}
-				items[string(pair.Key)] = KVPair{
+				items[string(pair.Key)] = &KVPair{
 					Key:    pair.Key,
 					Delete: true,
 				}
@@ -77,7 +77,7 @@ func genChangeSets(r *rand.Rand, n int) []ChangeSet {
 				i := r.Int63n(int64(len(lastChangeSet.Pairs)))
 				pair := lastChangeSet.Pairs[i]
 				if !pair.Delete {
-					items[string(pair.Key)] = KVPair{
+					items[string(pair.Key)] = &KVPair{
 						Key:   pair.Key,
 						Value: pair.Value,
 					}

nodedb.go

@@ -840,7 +840,7 @@ func (ndb *nodeDB) traverseStateChanges(startVersion, endVersion int64, fn func(
 
 		var changeSet ChangeSet
 		receiveKVPair := func(pair *KVPair) error {
-			changeSet.Pairs = append(changeSet.Pairs, *pair)
+			changeSet.Pairs = append(changeSet.Pairs, pair)
 			return nil
 		}
 

proto/buf.lock

@@ -0,0 +1,2 @@
+# Generated by buf. DO NOT EDIT.
+version: v1

proto/buf.md

@@ -0,0 +1,3 @@
+## IAVL
+
+Defines messages used by IAVL library, currently only `ChangeSet` and `KVPair`.

proto/buf.yaml

@@ -0,0 +1,10 @@
+version: v1
+name: buf.build/cosmos/iavl
+breaking:
+  use:
+    - FILE
+lint:
+  use:
+    - DEFAULT
+    - COMMENTS
+    - FILE_LOWER_SNAKE_CASE

proto/v1/changeset.proto

@@ -0,0 +1,12 @@
+syntax = "proto3";
+package iavl;
+
+message KVPair {
+    bool delete = 1;
+    bytes key = 2;
+    bytes value = 3;
+}
+
+message ChangeSet {
+    repeated KVPair pairs = 1;
+}