RW-559: Added tokens on RTask to support RTaskResult multiplexing.

Tokens can be used to "tag" an RTask with a client application specific token. These tokens can be then be used by a client application when an RTask completes to decide how best to process or direct the RTaskResult.

For example, if multiple "services" within a single application are using a single instance of RBroker, then each RTask submitted to that RBroker could be tagged with the name of the originating service.