Add PEX client (peer exchange plumbing command and automatic requests for p2p)

[kpcyrd]

There was a PEX server added recently (allowing remote peers to query a nodes peerdb), but this currently doesn't happen automatically, and there's also no plumbing command for this.

Both plumbing pex <addr> and plumbing dns-bootstrap should have an --add option to record the learned addresses to the peerdb.