Emulate schemata by using database as schema

[olavloite]

Google Cloud Spanner does not support multiple schemata in one database. This makes it impossible to use Cloud Spanner for applications that use a one-schema-per-tenant multi-tenancy strategy. The JDBC driver could emulate different schemata by considering different databases on the same instance as different schemata of one database. The setSchema(...) method and related methods would then change the current database used by the connection.
Note that in these cases, it would still be impossible to reference different schemata / databases in queries. Writing queries that join tables from different schemata would not be supported.