sqlexp: add in a message loop concept
sqlexp: add in a message loop concept

Drivers ususally hide the details of messages
from the server. Most clients don't care about details
or include complex SQL with multiple statements. Many still
do care about these details. Provide a way to dequeue messages
from the driver.

Relies on go1.9 ( https://golang.org/cl/38533 ).

Updates golang/go#19797

golang-sql/sqlexp