Consider making the gossiper component optional

[ralexstokes]

The gossiper linked is always created, even if config.relays is empty. If this config item is empty, everything still works although it does bring in some additional overhead to try to gossip various data which simply bails early upon finding there is no one to gossip to.

It would be nice if this entire feature is optional (and dependent on config.relays.len() != 0). It will avoid the (minimal) resource overhead in the case where an operator is not using the feature and more importantly make the codebase easier to reason about if an operator does not need the geo-distribution feature.