RW-550: Fixed priority task queue handling in RBrokerEngine.
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.
RW-559: Added tokens on RTask to support RTaskResult multiplexing.
RW-551: Added PooledTaskBroker grid fault tolerance.

PooledTaskBroker DeployR Grid Fault Tolerance

RTasks executing on a PooledTaskBroker can sometimes fail due to slot or node failures that occur at runtime on the DeployR grid. These types of RTask failures are candidates for automatic re-execution (without client application intervention) as they indicate a DeployR grid failure, not a failure of the RTask itself.

The new PooledTaskbroker grid fault tolerance feature:

1. Automatically re-executes RTasks that fail due to slot or node failures on the DeployR grid.

2. Automatically adjusts (reduces) the size of the pool to exclude failing RProject from causing further RTask failures.

These features combined deliver robust execution for RTasks and graceful degradation for the pool.