Fix races / finally remove global app for good (#7570)

* finally remove global app for good

* test compilation fixes

* fix races

* fix deadlock

* wake up write pump so it doesn't take forever to clean up

api4/system_test.go

@@ -6,15 +6,10 @@ import (
 	"testing"
 
 	l4g "github.com/alecthomas/log4go"
-	"github.com/mattermost/mattermost-server/app"
 	"github.com/mattermost/mattermost-server/model"
 	"github.com/mattermost/mattermost-server/utils"
 )
 
-func init() {
-	app.UseGlobalApp = false
-}
-
 func TestGetPing(t *testing.T) {
 	th := Setup().InitBasic().InitSystemAdmin()
 	defer th.TearDown()