port: add result set format and request type to port_sql

Result set serialization formats of DQL and DML queries are different:
the last one contains number of affected rows and optionally list of
autoincremented ids; the first one comprises all meta-information
including column names of resulting set and their types. What is more,
serialization format is going to be different for execute and prepare
requests. So let's introduce separate member to struct port_sql
responsible for serialization format to be used.

Note that C standard specifies that enums are integers, but it does not
specify the size. Hence, let's use simple uint8 - mentioned enum are
small enough to fit into it.

What is more, prepared statement finalization is required only for
PREPARE-AND-EXECUTE requests. So let's keep flag indicating required
finalization as well.

Needed for #2592