Handle prepared statements in NamedServerFilter

This is a request to allow NamedServerFilter to be able to route prepared statements to a specific server based on user and/or host. This currently works fine for SELECT, INSERT, UPDATE, DELETE, but does not work for prepared statements.

Per Markus, "the namedserverfilter will add a hint for the preparation but the execution of the prepared statement will not get a hint".

We see this when running sysbench against the NamedServerFilter, as the queries are not routed, unless you specify the --db-ps-mode=disable option.