server: eliminate possibly deadlock, peerConnected now async

This commit eliminates a possible deadlock (or repeated peer connection
failures) that can arise due to the [inbound|outbound]PeerConnected
methods holding the peer mutex too long. We now alleviate this
concurrency issue by calling s.peerConnected in an asynchronous manner.
This is safe as all operations within the method are themselves
goroutine-safe.

server.go

@@ -644,7 +644,7 @@ func (s *server) inboundPeerConnected(conn net.Conn) {
 	}
 	s.pendingConnMtx.RUnlock()
 
-	s.peerConnected(conn, nil, false)
+	go s.peerConnected(conn, nil, false)
 }
 
 // outboundPeerConnected initializes a new peer in response to a new outbound
@@ -669,7 +669,7 @@ func (s *server) outboundPeerConnected(connReq *connmgr.ConnReq, conn net.Conn)
 		return
 	}
 
-	s.peerConnected(conn, connReq, true)
+	go s.peerConnected(conn, connReq, true)
 }
 
 // addPeer adds the passed peer to the server's global state of all active