Investigate asynchronous fallback actions

[whitfin]

Fallbacks are currently executed inside the worker itself, which sucks when you have a long-running fallback. We should investigate how to deal with them asynchronously.

I think that maybe returning { :deferred, task } to the calling process is the way to go, and then you can just await on the task. The only problem with this is that async: true calls will never receive this reply... so it needs thinking about.

This would naturally be optional - it might be that you want to block the worker.

[whitfin]

Duh, I just realised that async: true calls are only on things which don't return values, so there's no fallback.

[whitfin]

This might not be viable due to the use case of transactions... Will revisit shortly.

[whitfin]

This will be made redundant by the changes coming for pooling in #37, so I'm going to close this. It can be revisited in future, but at this point the gain isn't big enough for the churn.