[MM-21378] Add mutex to model.Post to guard against race conditions on Post.Props (#13884)

* Add mutex to model.Post to guard against race conditions on Post.Props

* Rename mutex

* Add GetProp() method to Post

* Fix more tests

* Fix flaky test

Benchmarks:

BenchmarkPostPropsGet_indirect
BenchmarkPostPropsGet_indirect-2     	85026746	        13.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_indirect-4     	90273747	        13.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_indirect-8     	88324293	        13.0 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_indirect-16    	91427720	        13.1 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_direct
BenchmarkPostPropsGet_direct-2       	1000000000	         0.242 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_direct-4       	1000000000	         0.241 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_direct-8       	1000000000	         0.240 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsGet_direct-16      	1000000000	         0.241 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsAdd_indirect
BenchmarkPostPropsAdd_indirect-2     	 5602224	       203 ns/op	     336 B/op	       2 allocs/op
BenchmarkPostPropsAdd_indirect-4     	 5959496	       206 ns/op	     336 B/op	       2 allocs/op
BenchmarkPostPropsAdd_indirect-8     	 5833999	       205 ns/op	     336 B/op	       2 allocs/op
BenchmarkPostPropsAdd_indirect-16    	 5802493	       225 ns/op	     336 B/op	       2 allocs/op
BenchmarkPostPropsAdd_direct
BenchmarkPostPropsAdd_direct-2       	100000000	        11.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsAdd_direct-4       	100000000	        11.3 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsAdd_direct-8       	100000000	        11.6 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsAdd_direct-16      	99840794	        11.4 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsDel_indirect
BenchmarkPostPropsDel_indirect-2     	18824002	        61.9 ns/op	      48 B/op	       1 allocs/op
BenchmarkPostPropsDel_indirect-4     	19470736	        63.8 ns/op	      48 B/op	       1 allocs/op
BenchmarkPostPropsDel_indirect-8     	17640460	        65.3 ns/op	      48 B/op	       1 allocs/op
BenchmarkPostPropsDel_indirect-16    	18692962	        65.4 ns/op	      48 B/op	       1 allocs/op
BenchmarkPostPropsDel_direct
BenchmarkPostPropsDel_direct-2       	516257440	         2.34 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsDel_direct-4       	514865216	         2.43 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsDel_direct-8       	511330477	         2.37 ns/op	       0 B/op	       0 allocs/op
BenchmarkPostPropsDel_direct-16      	499504010	         2.38 ns/op	       0 B/op	       0 allocs/op

model/integration_action.go

@@ -368,8 +368,8 @@ func (r *SubmitDialogResponse) ToJson() []byte {
 
 func (o *Post) StripActionIntegrations() {
 	attachments := o.Attachments()
-	if o.Props["attachments"] != nil {
-		o.Props["attachments"] = attachments
+	if o.GetProp("attachments") != nil {
+		o.AddProp("attachments", attachments)
 	}
 	for _, attachment := range attachments {
 		for _, action := range attachment.Actions {
@@ -390,10 +390,10 @@ func (o *Post) GetAction(id string) *PostAction {
 }
 
 func (o *Post) GenerateActionIds() {
-	if o.Props["attachments"] != nil {
-		o.Props["attachments"] = o.Attachments()
+	if o.GetProp("attachments") != nil {
+		o.AddProp("attachments", o.Attachments())
 	}
-	if attachments, ok := o.Props["attachments"].([]*SlackAttachment); ok {
+	if attachments, ok := o.GetProp("attachments").([]*SlackAttachment); ok {
 		for _, attachment := range attachments {
 			for _, action := range attachment.Actions {
 				if action.Id == "" {
@@ -411,7 +411,7 @@ func AddPostActionCookies(o *Post, secret []byte) *Post {
 	retainProps := map[string]interface{}{}
 	removeProps := []string{}
 	for _, key := range PostActionRetainPropKeys {
-		value, ok := p.Props[key]
+		value, ok := p.GetProps()[key]
 		if ok {
 			retainProps[key] = value
 		} else {