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1. Continue to best-effort provide, but still return an error when we fail to
send provider records to the _best_ peers.
2. Continue returning the best peer's we've found in GetClosestPeers, but also
return an error to indicate that we didn't find the closest ones.

And fix the hang test.

fix Search/GetValue to be Kad compliant. Default quorum is now 0 which means do not abort the query early

* feat: Expose the ProviderManager through the IpfsDht struct
* feat: expose providers prefix so that we do not have to hard code it in other places
Co-authored-by: Aarsh Shah <aarshkshah1992@gmail.com>

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.