XXX Maybe there should be an option to allow this.

A ZEO client can run in disconnected mode, using data from
its cache, or in connected mode.  Several instance variables
are related to whether the client is connected.

_server: All method calls are invoked through the server
   stub.  When not connect, set to disconnected_stub an
   object that raises ClientDisconnected errors.

_ready: A threading Event that is set only if _server
   is set to a real stub.

_connection: The current zrpc connection or None.

_connection is set as soon as a connection is established,
but _server is set only after cache verification has finished
and clients can safely use the server.  _pending_server holds
a server stub while it is being verified.

Before this change, a client could start using a connection before
verification finished.  If verification took a long time, it could
even commit a new transaction using a mixing of old and new data.