Pool for keeping track of Spanner instances needed for connections.
When a connection is opened for a Google Cloud Spanner database, a Spanner object can
be opened in the background. The SpannerPool keeps track of which Spanner objects
have been opened by connections during the lifetime of the JVM, which connections are still
opened and closed, and which Spanner objects could be closed.
Call the method SpannerPool#closeSpannerPool() at the end of your application to
gracefully shutdown all instances in the pool.
Closes the default SpannerPool and all Spanner instances that have been opened
by connections and that are still open. Call this method at the end of your application to
gracefully close all Spanner instances in the pool. Failing to call this method will
keep your application running for 60 seconds after you close the last java.sql.Connection to Cloud Spanner, as this is the default timeout before the SpannerPool closes the unused Spanner instances.