gs1.1 extended validators

[wemeetagain]

Implements https://github.com/libp2p/specs/blob/master/pubsub/gossipsub/gossipsub-v1.1.md#extended-validators

Add extended topic validators and minimal refactoring to properly support it
Integrate scoring for validation (validation begin/reject/ignore), and successful message delivery

src/pubsub.js required some refactoring to support this feature. The main reason is that messages should first be checked for seenCache entries before running topic validation. Since the seenCache is currently on the gossipsub layer, and validation on the pubsub layer, there was some reworking to allow seenCache to checked before validation -- namely, moving validation from _processRpc, as a pre-step to _processRpcMessage, to instead run validation within _processRpcMessage. (See libp2p/js-libp2p-pubsub#50 and #92 for wrinkles still lingering)

This required some refactoring of the pubsub tests within this repo.

src/pubsub.js also got some slight refactoring around emitting messages.
_emitMessage now takes a single parameter (a InMessage), and is reused in both self publishing and publishing from received messages.
_processRpcMessage was extended with an additional method _publishFrom, which is meant to act as the "success case" for messages received from peers (step 7 in #92). This method is now extended on the gossipsub layer to broadcast messages to floodsub/mesh peers and support "message delivered" peer scoring.

[codecov-commenter]

Codecov Report

Merging #93 into gsv1.1 will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           gsv1.1      #93   +/-   ##
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  Coverage   81.81%   81.81%           
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  Files           1        1           
  Lines          11       11           
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  Hits            9        9           
  Misses          2        2           

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