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/

store/sqlstore/upgrade.go

@@ -753,7 +753,6 @@ func upgradeDatabaseToVersion519(sqlStore SqlStore) {
 
 func upgradeDatabaseToVersion520(sqlStore SqlStore) {
 	if shouldPerformUpgrade(sqlStore, VERSION_5_19_0, VERSION_5_20_0) {
-
 		sqlStore.CreateColumnIfNotExistsNoDefault("Bots", "LastIconUpdate", "bigint", "bigint")
 
 		sqlStore.CreateColumnIfNotExists("GroupTeams", "SchemeAdmin", "boolean", "boolean", "0")
@@ -775,6 +774,8 @@ func upgradeDatabaseToVersion521(sqlStore SqlStore) {
 func upgradeDatabaseToVersion522(sqlStore SqlStore) {
 	// TODO: Uncomment following condition when version 5.22.0 is released
 	// if shouldPerformUpgrade(sqlStore, VERSION_5_21_0, VERSION_5_22_0) {
+	// sqlStore.CreateColumnIfNotExistsNoDefault("Bots", "LastIconUpdate", "bigint", "bigint")
+	// sqlStore.AlterPrimaryKey("Reactions", []string{"PostId", "UserId", "EmojiName"})
 
 	// 	saveSchemaVersion(sqlStore, VERSION_5_22_0)
 	// }