Optimize reactions table (#13406)

* Optimize reactions table

Change reactions primary key to (PostId, UserId, EmojiName) so fetching
reactions for post will use primary key lookup instead of table scan.

* fix db version

* review fixes

* update database schema in scripts/

scripts/mattermost-mysql-5.0.sql

@@ -692,7 +692,7 @@ CREATE TABLE `Reactions` (
   `PostId` varchar(26) NOT NULL,
   `EmojiName` varchar(64) NOT NULL,
   `CreateAt` bigint(20) DEFAULT NULL,
-  PRIMARY KEY (`UserId`,`PostId`,`EmojiName`)
+  PRIMARY KEY (`PostId`,`UserId`,`EmojiName`)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4;
 /*!40101 SET character_set_client = @saved_cs_client */;