read-write connection is not available.  This refactors
ClientStorage.notifyConnected() into two functions, testConnection()
and notifyConnected().  testConnection creates the RPC stub and calls
the register() method; it returns the stub plus a flag indicating
whether this connection was preferred or sub-optimal.  If the
register() method raises ReadOnlyError, and the new option
read_only_fallback was true on the ClientStorage constructor, it is
retried with its read_only argument set to 1, and the stub is returned
with the sub-optimal flag.  notifyConnected() now receives the stub
returned by testConnection(), and starts the verification as before.

XXX The read_only_fallback  feature is not yet tested.

XXX More work is needed; when a suboptimal connection is used, the
ConnectThread must stay around trying to get a preferred connection,
and then it must switch connections on the ClientStorage (how???).