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* fix: ensure broadcast when peer becomes unavailable and all other peers sent DONT_HAVE for CID

* fix: lint warnings

* refactor: simplify session want sender DONT_HAVE list

* fix: flaky test

* test: add session exhausted wants test

* docs: improve sessionWantSender processAvailability docs

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

feat: process response message blocks as a batch

Better estimate latency per peer by tracking cancellations

When fetching optimized peers from the peer manager, return an optimization rating, and pass on to
request splitter

BREAKING CHANGE: interface change to GetOptimizedPeers and SplitRequests public package methods

Re-setup provider search delay and rebroadcast delay on a per bitswap instance basis

on a tick, do not keep searching for providers for the same block. instead rely on a periodic search
for more providers. (which will run no matter what, even w/o ticks, to optimize found providers).
also backoff tick time to reduce broadcasts.

fix #95, fix #107

Improve stability of tests for Session and SessionPeerManager

fix #61

Encapsulate functions for readability, and move code for understanding

Move the job of splitting requests to its own package

As soon as peers appear, consume all of the want budget

Reduce duplicates through splits of requests

fix #43

Add a unit test for session package