Previously, stdlib.RegisterConnConfig would sometimes reuse the same connection
string for different ConnConfig options (specifically, it happened when a connection
was open and then closed, and then a new, different connection was opened). This
behavior interferes with callers that expect that two connections with the same data
source name are connecting to the same backend database in the same way.
This fix updates stdlib.RegisterConnConfig to use an incrementing sequence
counter to uniquify all returned connection strings.
Fixes#947
Andrew Kimball
3ab8941921