Initial version of the polite-grandpa networking protocol (#2110)

* Consensus status packet

* Allow for repropagation after status

* More generic gossip

* add a basic view struct and gossip module

* move gossip stuff to the gossip module

* integrate view into gossip

* some reshuffling

* alter rules for keeping one commit at a time in view

* Allow sending addressed messages

* don't cast outgoing votes if we know that we voted before

* Handle one hop messages

* initial run at polite grandpa

* build WASM

* handle neighbor messages

* refactor validator's internals into an Inner struct

* gossip only knows to keep or discard messages. optimize should_send_to

* Periodic rebroadcast

* implement `should_send_to` and message_expired

* track peers' best received commit height

* Pass peer id to topic steam

* kill rebroadcasting network

* Notify about existing peers

* clean up network APIs a bunch

* implement gossip::send_message for direct messages

* refactor network trait

* implement gossip::send_message for direct messages

* get all non set-change tests passing

* treat unknown rebroadcasts as broadcasts

* get all other main tests passing

* remove unimplemented test

* everything compiles

* treat unknown rebroadcasts as broadcasts

* Rebradcast interval

* Apply suggestions from code review

Style

Co-Authored-By: arkpar <arkady.paronyan@gmail.com>

* Style

* some module docs

* address some grumbles + docs

* allow rebroadcast every few minutes

* send_topic && generic context

* some tests for view change

* more grumbles & tests

* use send_peer