add GossiperMaker, GetGossip(), and UTs

[britterm]

This PR adds support for custom surrogate Gossiper implementations, as well as the ability to access surrogate Gossip channels.

If a GossiperMaker is not set, or the assigned GossiperMaker returns nil, the default surrogate implementation will be used for that channel.

This change will allow mesh clients to dynamically provision Gossipers for channels instead of defining them all up-front. An example use case would be to use channels as dynamic "namespaces" for data feeds. The data in the feeds could all be the same, or the channel name could indicate what type of data is in the feed. The GossiperMaker could assign different Gossipers to each channel as they are discovered. These channels would be retrievable and usable with the new GetGossip() method on the router.

~/Development/workspace/git/forks/mesh master
❯ go build

~/Development/workspace/git/forks/mesh master
❯ go test
->[|02:00:00:02:00:00(nick)]: connection added (new peer)
->[|02:00:00:02:00:00(nick)]: connection fully established
->[|01:00:00:01:00:00(nick)]: connection added (new peer)
->[|01:00:00:01:00:00(nick)]: connection fully established
->[|03:00:00:03:00:00(nick)]: connection added (new peer)
->[|03:00:00:03:00:00(nick)]: connection fully established
->[|02:00:00:02:00:00(nick)]: connection added (new peer)
->[|02:00:00:02:00:00(nick)]: connection fully established
->[|01:00:00:01:00:00(nick)]: connection added
->[|01:00:00:01:00:00(nick)]: connection fully established
->[|03:00:00:03:00:00(nick)]: connection added
->[|03:00:00:03:00:00(nick)]: connection fully established
->[|03:00:00:03:00:00(nick)]: connection deleted
->[|03:00:00:03:00:00(nick)]: connection deleted
->[|02:00:00:02:00:00(nick)]: connection added (new peer)
->[|02:00:00:02:00:00(nick)]: connection fully established
->[|01:00:00:01:00:00(nick)]: connection added (new peer)
->[|01:00:00:01:00:00(nick)]: connection fully established
->[|02:00:00:02:00:00(nick)]: connection added (new peer)
->[|02:00:00:02:00:00(nick)]: connection fully established
->[|03:00:00:03:00:00(nick)]: connection added (new peer)
->[|03:00:00:03:00:00(nick)]: connection fully established
->[|02:00:00:02:00:00(nick)]: connection added (new peer)
->[|02:00:00:02:00:00(nick)]: connection fully established
->[|01:00:00:01:00:00(nick)]: connection added (new peer)
->[|01:00:00:01:00:00(nick)]: connection fully established
->[|02:00:00:02:00:00(nick)]: connection added (new peer)
->[|02:00:00:02:00:00(nick)]: connection fully established
->[|03:00:00:03:00:00(nick)]: connection added (new peer)
->[|03:00:00:03:00:00(nick)]: connection fully established
PASS
ok  	_/Users/britterm/Development/workspace/git/forks/mesh	0.172s