neighborhood suggester configuration option

[istae]

a new configuration option, neighborhood-suggester, is a web address, when queried, returns the most optimal neighborhood to mine a new node for. (see https://api.swarmscan.io/v1/network/neighborhoods/suggestion)

New nodes mining their overlay for the first time can utilize the suggester to target a specific neighborhood.
If there is already a target-neighborhood option supplied, the suggester is ignored.

By default, the suggester should point to the swarmscan api.