PLT-3695 refactoring to use sql_upgrader (#3788)

* PLT-3695 unitying upgrade support

* PLT-3695 refactoring to use sql_upgrader

* Fixing unit test

* Adding 3.3 to 3.4

* Fixing upgrading

* Seperating DB schema from version.go

* Fixing upgrade cmd

* Moving to const for exit codes

* remving old unused code

store/sql_channel_store.go

@@ -45,9 +45,6 @@ func NewSqlChannelStore(sqlStore *SqlStore) ChannelStore {
 	return s
 }
 
-func (s SqlChannelStore) UpgradeSchemaIfNeeded() {
-}
-
 func (s SqlChannelStore) CreateIndexesIfNotExists() {
 	s.CreateIndexIfNotExists("idx_channels_team_id", "Channels", "TeamId")
 	s.CreateIndexIfNotExists("idx_channels_name", "Channels", "Name")