Allow database client to notice schema changes

At the moment the various implementations of database drivers and the database client cache the information about the schema used by the database, and therefore if another client, or something else, changes the schema, or cleanups/initializes the database, the client doesn't notice that. Right now that is a problem mostly for the command-line tool tests. They have both a Client object, and invoke the tools which make the schema modifications at the same time. Therefore they can't notice the changes made by the tools with their Client object. The Cloud Functions have this problem too, and need to be stopped for a database upgrade, but that's generally unavoidable in case of backwards-incompatible changes, as those are non-atomic.

Consider adding an interface to refresh/reinitialize/recreate the client on request, so that any schema changes are picked up without having to explicitly recreate the client from potentially locally-unavailable parameters.