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This makes reading the docs much easier as it's clear what's "private" and
what's not.

fixes #238

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

Seperate want list into differentiated types - session tracking and regular

fix #13

Move refcnt tracking from the messagequeue to the peermanager, where it's relevant

repace get/set with getOrCreate to keep operations atomic

Breakup Startup function so that wantlists are not sent with each call to SendMessage

If wantlist changes are present, send them immediately on startup, rather than as a seperate message

fix race conditions while setting up wantlists by creating peer queues on demand

BREAKING CHANGE: PeerManager SendMessage signature changed

fix #51

Finishing adding comments to WantManager and PeerManager, refactor message structure for type
safety, add sending messages test

Seperates the functions of tracking wants from tracking peers
Unit tests for both peer manager and want manager
Refactor internals of both to address synchonization issues discovered
in tests