Bithack

*BitHack* makes a few changes to the way our BitSwap implementation work. From an API standpoint, the biggest change is the addition of `GetBlocks`.  `GetBlock` has been changed to be a special case call of `GetBlocks`. When `GetBlocks` is called, the keys that are requested are sent to another goroutine through a channel, where the actual requests are handled.

When a set of keys are requested, BitSwap will add them all to the wantlist, and the find providers for the first key in the list. It will then send the wantlist to those found providers.
The main bitswap loop now contains a 'timeout ticker', when it ticks, bitswap will find providers for every single key left on the wantlist and send the wantlist out to them, this is the 'plan B' as we hope that our initial request succeeds, saving us the bandwidth. This operation could be optimized to ensure only sending wantlist to a peer once per tick.

`ReceiveMessage` also was changed. Previously it would *always* return a message, which looked to me like it would cause an infinite loop of messaging between a set of peers. Im surprised we never experienced anything bad because of it.

To take advantage of this new behaviour, I modified the DAGReader to use GetBlocks to retrieve blocks for reading.
Allow me clarify my perspective. 

There's nothing inherently _wrong_ about only sending the first key.

But notice that if we only send the first key, there's no guarantee that this will result in a successful request.

If we choose this behavior here, it must be married with a timely broadcast that performs a full sweep.

Then, we are relying on timely broadcast to reliably satisfy requests. In this case, the `key[0]` fetch is merely an optimization. It's a heuristic based on a belief that a peer that can satisfy the first key _might_ be able to satisfy the rest of the request. 

What's important to me is that we _recognize_ that this is the choice we're making.

I was uncomfortable when the `key[0]` optimization was paired with the `wantlist[rand]` optimization. That didn't make any sense.