enforce timeout on DYHB queries?

[warner]

When a file is uploaded via the helper, the first thing the helper does is to ask all the storage servers if they already have the shares. If one of those servers has recently been silently lost (a laptop that was suspended, or a network connection that's been severed), the query can take up to 15 minutes to time out.

It might be nice to accelerate this process by establishing an upper bound on the amount of time that we'll wait for a response, perhaps 10-15 seconds. We could use similar measures on share transmission, specifically download (since we have plenty of peers to choose from).