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* feat: small optimizations

* feat: move broadcast wantlist into the peermanager

This deduplicates some state and allows us to do less book-keeping for broadcast
wants. We should probably rename the PeerManager to the WantManager and rename the
WantManager to something else.

* fix: lint warnings

* fix: minimize time holding wantlist lock

Instead of holding the lock the entire time we prepare a message, hold the lock
while we retrieve the wantlist entries, process the entries without the lock,
retake the lock, then mark entries as sent.

This means:

1. We never sort entries while holding the lock.
2. We allocate exactly three times while holding the lock (once per entry list).

* fix: address code review

Instead of copying these slices, we can just reuse them.

In case we're sending a _lot_ of wants:

* Prioritize cancels.
* Then targeted wants.
* Finally broadcast wants.

This removes one goroutine per peer which tends to be a pretty big deal. This
brings go-ipfs down from 5.5 to 4.5 goroutines per peer.

* 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 it easy to send immediately if we wait too long and/or if we have enough
to send.
* Is significantly more efficient than the debounce library as it doesn't spin
off a bunch of "after" timers.

fixes #245

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