User option to advertise relay addresses

#### Type: Enhancement

#### Description:

We currently filter relay addresses by default, with thre rationale that it is an experimental feature.
We do want to make it easy for node operators to use however, especially when they know a priori that
they are behind an impenetrable NAT.

We should implement a configuration option, Swarm.AdvertiseRelayAddress with default of false, that
disables the filtering of relay addresses. When the option is set, the unspecific p2p-circuit address will not
be filtered, which will allow peers to automatically attempt dialing through known relays.