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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