database/sql: on Tx rollback, retain connection if driver can reset session

database/sql: on Tx rollback, retain connection if driver can reset session

Previously the Tx would drop the connection after rolling back from
a context cancel. Now if the driver can reset the session,
keep the connection.

Change-Id: Ie6a3124275632787629844d91a06bb2e70cc060b
Reviewed-on: https://go-review.googlesource.com/c/go/+/216241
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>