swarm_manager: freely connect to nodes with a p2p link

setKnownNodes will add new nodes that may be already connected
with a TCP link, but maintainBuckets may not choose those nodes,
delaying the bootstrap.
If we detect a new node that we're already connected with, we can
add it in the list of nodes we want to connect with.

Change-Id: I422a32495693e1e2d925a3af127a4a59903c1833