Retry Spanner schema creation and shutdown on ultimate failure

[hseeberger]

See https://github.com/lightbend/cloudstatemachine/issues/2083.

This PR improves on the so far very naive approach to check and create the database schema for Google Cloud Spanner:

• Several admin calls are unified into single a one
• Schema check/creation is retried a configurable number of times after a configurable delay
• After ultimate failure the actor system is shutdown (for k8s to react upon)