Add async.Abort

[ghost]

The following should immediately cease execution of the currently executing task, but should not output an error level log and no subsequent actions should be taken.

raise async.Abort()

[johnlockwood-wf]

How will we want run_job to handle an async abort if it is part of a larger job? Send an abort signal to all others? Treat that task as done with no result and trigger a check for job completion? Maybe do that and record it as an error or warning job status entity.

[ghost]

@johnlockwood-wf We're not sure what the right behavior is in that case. We discussed it, and decided to just implement this for async's at this time, and to consider the context case independently.