Add 'firstCompleted' and 'firstCompetedFuture' to 'asyncfutures2'

[zah]

These are useful alternatives to the race helper that make it easy to implement "first successful response" strategies.

Currently, the Nimbus VC is trying to implement such a strategy, but it has the issue that if some of the servers responds too quickly with an error, the successful responses that might complete later will be ignored.