GitLab

This commit extends the bitswap protocol with two additional wantlist properties:

* WANT_HAVE/HAVE: Instead of asking for a block, a node can specify that they
  want to know if any peers "have" the block.
* WANT_HAVE_NOT/HAVE_NOT: Instead of waiting for a timeout, a node can explicitly
  request to be told immediately if their peers don't currently have the given
  block.

Additionally, nodes now tell their peers how much data they have queued to send
them when sending messages. This allows peers to better distribute requests,
keeping all peers busy but not overloaded.

Changes in this PR are described in: https://github.com/ipfs/go-bitswap/issues/186