allow service.run to manage its own receipts

[britram]

if mplane.scheduler.Service.run() returns a receipt (instead of a result), the framework should allow it to manage its own state, i.e., invoking run() with the redemption returned from the receipt, rather than doing a lookup in its pending jobs. Required to allow redirection for RIPE Atlas integration, wherein a specification invokes an external measurement which itself has an identifier (Atlas MSM) which can be used as a parameter for later retrieval.

[muemart]

I implemented something similar in my fork, but with a difference. Rather than redeem the new receipt, the client just tries to invoke a matching capability (and basically discards the receipt). Works for my purposes, but feels a bit like a hack.