[GCP] Clean up Spanner connections on shutdown

[haydentherapper]

Description

Tessera manages the connections to Spanner and GCS. When shutting down a log that uses the Tessera library, there is no way to close GCP connections. We would like a way to invoke the Spanner client Close() for dbPool. It's not strictly required to call the GCS client Close() for gcsClient as noted in the documentation, but it might not be a bad idea just in case there's a future change.

It would be ideal if this could be handled automatically as part of invoking the Shutdown lifecycle function.

The other storage backends may need similar handling for a graceful shutdown, but I didn't look into any others. Maybe a Driver can provide a Shutdown function which is invoked by terminator.Shutdown?