Configurable Heartbeat Interval

[ZenGround0]

I'd like to be able to configure the gossipsub router's heartbeat interval. This is motivated by setting up local testing networks. The current interfaces don't give me a way to block until the mesh overlay is setup which I need to block on before publishing because of the behavior in #211. This mesh setup doesn't happen until the first heartbeat (the initial delay doesn't help because it is sometimes called before the first Join), and the default interval of 1 second is a bit too long for tests.

[vyzo]

The heartbeat interval is already configurable through the GossipSubHeartbeatInterval variable!