Transpile "use db".

The exasolrouter catches COM_INIT_DB packets and translates it into "OPEN SCHEMA db". However, if "use db" is sent as SQL, it will reach Exasol where it will cause an error, even if the query is run through SQLGlot, since SQLGlot does not transpile it.
Catching "use db" is trivial, so the Exasol router should transpile into "OPEN SCHEMA db".