Cloud Spanner JDBC connections do not maintain a physical connection to
Cloud Spanner, but are merely a wrapper around the underlying Java
client library. This again uses a pool of gRPC channels to communicate
with Cloud Spanner. This means that a single JDBC connection will never
lose its network connection with Cloud Spanner, and checking whether it
is valid or not by executing a query every time is not useful. Instead,
the check should:
1. Verify that a connection has successfully been established with Cloud
   Spanner. The result should be cached for all JDBC connections using
   the same Cloud Spanner client library instance.
2. Verify that the connection has not been closed.

The above can be achieved by checking that the dialect of the database
that the connection is connected to has been successfully fetched. This
result is cached in the client library, and being able to get that means
that there has been a valid connection.

This means that the isValid method will still return true if the network
connectivity has been lost completely between the client and Cloud
Spanner. However, this check is mostly used by connection pools to
determine whether a connection is safe to be handed out to an
application, and when all network connectivity has been lost, this will
apply to all JDBC connections and not just one, meaning that the check
is void.

The original isValid check can be enabled by setting the System property
spanner.jdbc.use_legacy_is_valid_check to true or setting the
Environment variable SPANNER_JDBC_USE_LEGACY_IS_VALID_CHECK to true.

Fixes #1443