Provide TemporaryTableDDL to mange temporary table ddl opeartions.

[lcwangchao]

We have too many temporary table states in session.variables and they are exposed to users directly. For example, when dropping a local temporary table, the ddl executor should invoke sessVars.LocalTemporaryTables.RemoveTable first and then clear the table's data in sessVars.TemporaryTableData.

To decouple those codes, we should introduce a package to manage these operations and provide unified APIs. For the ddl operations this pr introduces a new interface TemporaryTableDDL to do the ddl things. It hides detail states in session.variable and the users should not care about it.