Something is broken with the cache.

Cherry-picks https://github.com/ipfs/go-ipfs/pull/6488

libp2p: moves discovery after host listen

Fix: flush pin on block put

pin: fix concurrent map access race

Fix: ensure pin rm takes a lock

1. Build issue with badger due to deleted tag.
2. High CPU issue in bitswap under high load.
3. Significant throughput regression in bitswap.

release 0.4.21

Add 0.4.21 CHANGELOG

Co-Authored-By: Michael Avila <michaelavila@users.noreply.github.com>

Co-Authored-By: Dominic Della Valle <ddvpublic@Gmail.com>

Co-Authored-By: MollyM <momack2@users.noreply.github.com>

WIP. Please either add or mention in comments what should be featured.

License: MIT
Signed-off-by: Jakub Sztandera <kubuxu@protonmail.ch>

fix: use http.Error for sending errors

This sets a few headers that prevent browsers from misinterpreting the error
text.

core: call app.Stop once

explain what dhtclient does

release: v0.4.21-rc3

This time, we'll actually build it.