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.
In this case, if it's an ErrNotFound, then we use the remote. otherwise, something unexpected went wrong and we should bubble up. Actually, the code is currently:

```Go
block, err := s.Blockstore.Get(k)
if err == nil {
  return block, nil
  // TODO be careful checking ErrNotFound. If the underlying
  // implementation changes, this will break.
} else if err == ds.ErrNotFound && s.Remote != nil {
  log.Debug("Blockservice: Searching bitswap.")
  blk, err := s.Remote.GetBlock(ctx, k)
  if err != nil {
    return nil, err
  }
  return blk, nil
} else {
  log.Debug("Blockservice GetBlock: Not found.")
  return nil, ErrNotFound
}
```

Which doesn't bubble up the error. I suggest:

```Go
block, err := s.Blockstore.Get(k)
if err == nil {
  log.Debug("Blockservice GetBlock: Found block locally.")
  return block, nil
}

if err != ds.ErrNotFound {
  log.Debug("Blockservice GetBlock: Unexpected error:", err)
  return nil, err
}

if s.Remote == nil {
  log.Debug("Blockservice GetBlock: Not found (no bitswap).")
  return nil, ErrNotFound
}

log.Debug("Blockservice: Searching bitswap.")
blk, err := s.Remote.GetBlock(ctx, k)
if err != nil {
  return nil, err
}
return blk, nil
```