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fixes #274

When the routing table is empty, `Run` would fail but _not_ close the
process (leaking some query goroutines). This patch fixes this in multiple
places by:

1. Not starting queries with no peers.
2. Failing queries with no peers earlier.

This reverts commit 7e68ac3c.

Also, de-duplicate some logic.

fixes #177

fixes #172

err is nil in this case

Also, double check the key (should be impossible but the check is cheep).

Note: this does mean that the DHT won't work with peer keys by default and that
the constructor signature changes. Given all the changes that'll come with the
libp2p refactor, I don't feel too bad about this.

GetValues was very DHT specific so the routing interface has been updated to
remove that function. Instead, it has introduced general-purpose options.

This is a minimal alternative to #141 to avoid bundling too many changes
together.