fix bootstrap: add node after successful handshake

Previous version was very conservative about adding nodes to the
routing table, requiring a full request/response message exchange
initiated by us.
Here we also add the node as 'seen' if the whoareyou/handshake
exchange was successful.
This significantly speeds up bootstreap from 0

Signed-off-by: Csaba Kiraly <csaba.kiraly@gmail.com>

libp2pdht/private/eth/p2p/discoveryv5/node.nim

@@ -28,7 +28,8 @@ type
     address*: Option[Address]
     record*: SignedPeerRecord
     seen*: bool ## Indicates if there was at least one successful
-    ## request-response with this node.
+    ## request-response with this node, or if the nde was verified
+    ## through the underlying transport mechanisms.
 
 func toNodeId*(pk: keys.PublicKey): NodeId =
   ## Convert public key to a node identifier.

libp2pdht/private/eth/p2p/discoveryv5/transport.nim

@@ -144,6 +144,9 @@ proc receive*(t: Transport, a: Address, packet: openArray[byte]) =
         # on the next revalidation, one could spam these as the handshake
         # message occurs on (first) incoming messages.
         if node.address.isSome() and a == node.address.get():
+          # TODO: maybe here we could verify that the address matches what we were
+          # sending the 'whoareyou' message to. In that case, we can set 'seen'
+          node.seen = true
           if t.client.addNode(node):
             trace "Added new node to routing table after handshake", node
   else: