RW-579: RBroker server endpoint validation on initialization.

RBroker instances accept but do not validate or test the DeployR server endpoint passed on their RBrokerConfig. For anonymous RBroker instances this means invalid server endpoints are not detected until the first attempt to execute an RTask, potentially long after RBroker initialization has completed. This is not conducive to building robust client applications.

Authenticated RBroker instances use the server endpoint at initialization (in an attempt to authenticate with the server) so are not prone to this issue.