More tests and admin server fixes

[maxdml]

• QueueOptions made public
• More comprehensive recovery test (recovers more workflows and check steps status)
• Check that steps return a cancelled error after the workflow was cancelled
• Admin server:
  • Always "stringify" non nil workflow output/input
  • Filter out queue tasks by status (pending/enqueued) on the list queues endpoint
• Have RetrieveWorkflow return a handle interface