This commit adds a new interface the to discovery package:
NetworkPeerBootstrapper. The NetworkPeerBootstrapper interface is meant
to be used to bootstrap a new peer joining the network to the set of
existing active peers within the network. Callers are encouraged to
utilize several boostrappers in series as redundant sources of
information. The MultiSourceBootstrap function will takes a set of
boostrappers, and compose their outputs into a single unified set of
addresses.
Two concrete implementations of the NetworkPeerBootstrapper interface
have been added as a part of this commit: the ChannelGraphBootstrapper
and the DNSSeedBootstrapper. The former will utilize the authenticated
node advertisements within the calling nodes view to boostrap new
connections. The latter will use a set of BOLT-0010 compliant DNS seeds
to query. This DNS seeding more will likely be used by nodes initial
joining the network, as they may not yet have the channel graph as they
haven’t connected to any peers.
