gs1.1 adaptive gossip

[wemeetagain]

Implements https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#adaptive-gossip-dissemination

• Closely follows the go-libp2p-pubsub implementation
• Adaptive gossip dissemination in _emitGossip
• fixes up the fanout peer handling in heartbeat
  • includes bugfix in the heartbeat where we were deleting fanout peers who are still in the topic (yikes)
• Add hasGossipProtocol utility function, replace where appropriate

[codecov-commenter]

Codecov Report

Merging #94 into gsv1.1 will increase coverage by 1.81%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           gsv1.1      #94      +/-   ##
==========================================
+ Coverage   80.00%   81.81%   +1.81%     
==========================================
  Files           1        1              
  Lines          10       11       +1     
==========================================
+ Hits            8        9       +1     
  Misses          2        2              

Impacted Files	Coverage Δ
test/utils/index.js	81.81% <0.00%> (+1.81%)	⬆️

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[vasco-santos]

LGTM!