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.
> Instead of breaking down bitswaps responsibilities, I want to move more logic into the strategies. Bitswap itself should just be a frame to run a strategy

The scope of what the strategy has power over is defined at the protocol level. Currently it's just `func (Strategy) ShouldSend(Peer, Ledger) bool`. At this time, nothing else is part of the protocol. We can expand it over time, but this has to be an explicit decision. I agree more should be included (like what blocks to get as assets to trade, who to connect to, etc). 

> If you're envisioning something different, please propose a new term to distinguish between the entity described in 3.4.2 and what you have in mind.

I think @whyrusleeping means precisely the same thing.