lnd: pass through neutrino MaxPeers config

This enables users to set the config. We check that the currently set
value of MaxPeers is non-zero.

sample-lnd.conf

@@ -840,9 +840,7 @@
 ; neutrino.connect=
 
 ; Max number of inbound and outbound peers.
-;
-; NOTE: This value is currently unused.
-; neutrino.maxpeers=
+; neutrino.maxpeers=8
 
 ; Add a peer to connect with at startup.
 ; neutrino.addpeer=