[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

api4/command_test.go

@@ -616,7 +616,6 @@ func TestExecuteGetCommand(t *testing.T) {
 	assert.True(t, len(commandResponse.TriggerId) == 26)
 
 	expectedCommandResponse.TriggerId = commandResponse.TriggerId
-	expectedCommandResponse.Props["from_webhook"] = "true"
 	require.Equal(t, expectedCommandResponse, commandResponse)
 }
 
@@ -675,9 +674,7 @@ func TestExecutePostCommand(t *testing.T) {
 	assert.True(t, len(commandResponse.TriggerId) == 26)
 
 	expectedCommandResponse.TriggerId = commandResponse.TriggerId
-	expectedCommandResponse.Props["from_webhook"] = "true"
 	require.Equal(t, expectedCommandResponse, commandResponse)
-
 }
 
 func TestExecuteCommandAgainstChannelOnAnotherTeam(t *testing.T) {