upload should succeed as soon as the servers-of-happiness criterion is met

[davidsarah]

The performance of upload could be improved by returning success to the client as soon as the servers-of-happiness criterion is met, rather than when all shares have been placed. The storage client would continue to upload the remaining shares asynchronously.

This should have no impact on preservation or availability, because once the happiness criterion has been met, we know that we are going to return success to the client even if no further shares can be placed.

Upload performance should be improved in a similar way to #928's improvement of download performance (although less dramatically, since with the default parameters, we will be returning after shares have been uploaded to the fastest 7 servers, rather than after shares have been downloaded from the fastest 3 servers as for #928).