fix: handle SET ROLE properly

[flaneur2020]

this PR handles the SessionState as a whole:

1. on starting each query, it send the SessionState to the query instance
2. query instance responses a new SessionState to the client, and the client just save it
3. client pick the reponded SessionState to the next query

this may help reduce the cognize load when adding things into the SessionState, do not need handle_session() and make_session() with understanding of the fields, all we need is to pass it as a whole.