* mcache fixes
* remove timed cache - the window shifting already removes old messages
* ref -> object
* avoid unnecessary allocations with `[]` operator
* simplify init
* fix several gossipsub/floodsub issues
* floodsub, gossipsub: don't rebroadcast messages that fail validation
(!)
* floodsub, gossipsub: don't crash when unsubscribing from unknown
topics (!)
* gossipsub: don't send message to peers that are not interested in the
topic, when messages don't share topic list
* floodsub: don't repeat all messages for each message when
rebroadcasting
* floodsub: allow sending empty data
* floodsub: fix inefficient unsubscribe
* sync floodsub/gossipsub logging
* gossipsub: include incoming messages in mcache (!)
* gossipsub: don't rebroadcast already-seen messages (!)
* pubsubpeer: remove incoming/outgoing seen caches - these are already
handled in gossipsub, floodsub and will cause trouble when peers try to
resubscribe / regraft topics (because control messages will have same
digest)
* timedcache: reimplement without timers (fixes timer leaks and extreme
inefficiency due to per-message closures, futures etc)
* timedcache: ref -> obj
Jacek Sieka