update gossip (part 1)

[turuslan]

• update gossipsub #297
• compare with rust-libp2p (old floodsub) (gossipsub)
• test gossip peer against mock streams/peers
• respect peer protocol version
• (rust) separate MessageCache (id and message data, gossip ihave, reply to iwant) and DuplicateCache (id only)
• (rust) gossip in heartbeat instead of publish/forward, may gossip same message to more peers in more heartbeats
• fanout peers (publish when we are not subscribed, like mesh but without explicit graft)
• send prune with backoff
• flood publishing (not same as floodsub)
• fix Connectivity/Stream/GossipCore shared_ptr cycle/leak.
• remove hasTopic, add isSubscribed
• remove empty, add isUsed
• simplify stream write batch
• fix MessageReadWriterUvarint returned null for zero size message
• stubs: ExplicitPeers, Score