gs1.1 peer exchange

[wemeetagain]

Implements https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#prune-backoff-and-peer-exchange

Follows go-libp2p-pubsub implementation closely

Depends on libp2p/js-libp2p#683
As noted in a comment there, we now require the libp2p repo as a full-fledged dependency, due to needing to openAndCertify the raw envelope.

[codecov-commenter]

Codecov Report

Merging #100 into gsv1.1 will increase coverage by 5.88%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           gsv1.1     #100      +/-   ##
==========================================
+ Coverage   75.93%   81.81%   +5.88%     
==========================================
  Files          22        1      -21     
  Lines        1342       11    -1331     
==========================================
- Hits         1019        9    -1010     
+ Misses        323        2     -321     

Impacted Files	Coverage Δ
src/score/computeScore.js
src/score/messageDeliveries.js
src/messageCache.js
src/utils/shuffle.js
src/index.js
aegir.js
src/score/index.js
src/utils/index.js
src/message/rpc.proto.js
src/score/constants.js
... and 13 more

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[wemeetagain]

@vasco-santos This is ready for re-review. Codecov is giving an ❌ but we'll tackle the tests in another PR (in progress 😃)