feat(spanner): Add option to disable closing spanner clients

[jtwatson]

Provide an option to prevent the provided *spanner.Client and *database.DatabaseAdminClient from being closed when migrate closes Spanner

Spanner does not have the same concepts as PostgreSQL and MySQL, where it makes sense to use a WithConnection() function as the signature would be identical to that of WithInstance(). It could appear that they are interchangeable when really one closes the connection, and the other does not.

I chose to take a simple path of adding an option to the Config, DoNotCloseSpannerClients. This is a very simple change that is clear about its effect and does not change the default behavior of WithInstance().

But this is not in line with the other drivers implementing this same feature, so feels a little inconsistent.

Maybe someone has a better idea.

See the following thread for reference:

• How it was implemented for PostgreSQL: Add WithConnection to Postgres similar to MySQL. #659
• how it was implemented for MySQL: Database driver closes the connection pool #578 Added a method to create a mysql database from a connection object #583

[coveralls]

coverage: 59.21% (-0.02%) from 59.226%
when pulling 9c48518 on jtwatson:feature/spanner-client-reuse
into 0d41589 on golang-migrate:master.

[jtwatson]

@dhui @Fontinalis, can you take a look at this PR? I am hoping to get some feedback on the provided feature.