Adds support for Google Spanner as a data store for messages.
Using sqlstore.go as a starting point, the SQL specific bits were simply replaced with Spanner equivalents.
Testing will be a bit tricky, as you need a gcloud setup to test against a "real" Spanner DB. There is a Docker Spanner emulator, but it also relies on gcloud. This library looks promising for Spanner tests: https://github.com/googleapis/google-cloud-go/tree/master/spanner/spannertest
Maybe a further discussion should be had around tests for Spanner?