gs1.1 flood publishing

[wemeetagain]

Builds off #87
Implements https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#flood-publishing

Add floodPublish option, defaulted to true
With flood publishing on:
Self-published messages should be sent to all peers with scores above the publish threshold

With flood publishing off:
Self-published messages should be sent using gossipsub 1.0 functionality

[codecov-commenter]

Codecov Report

Merging #89 into gsv1.1 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           gsv1.1      #89   +/-   ##
=======================================
  Coverage   80.00%   80.00%           
=======================================
  Files           1        1           
  Lines          10       10           
=======================================
  Hits            8        8           
  Misses          2        2           

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[wemeetagain]

I'm going to update getGossipPeers in a separate PR first for use here.
Looking at the go implementation, the gossipsub v1.0 behavior is also tweaked, uses the publish threshold to select mesh peers. This (and other) features will benefit getGossipPeers to be updated to suit

[vasco-santos]

This looks good! #90 should get in first