Send ParameterStatus on SET statements

[sunng87]

In vanilla postgresql, a ParameterStatus will be sent upon successful set statement, just like how startup process performed. We will need to adopt this behaviour in our hook for set_show statement.

To send ParameterStatus, we should use pgwire API by calling client.send directly. A reference can be found at: https://github.com/sunng87/pgwire/blob/master/src/api/auth/mod.rs#L310