config+lnd: make WebSocket interval/wait configurable

To make it possible to adapt the WebSocket ping interval and pong wait
time to custom situations, we add new configuration flags to customize
them.

sample-lnd.conf

@@ -184,6 +184,15 @@
 ; Disable TLS for the REST API.
 ; no-rest-tls=true
 
+; The ping interval for REST based WebSocket connections, set to 0 to disable
+; sending ping messages from the server side. Valid time units are {s, m, h}.
+; ws-ping-interval=30s
+
+; The time we wait for a pong response message on REST based WebSocket
+; connections before the connection is closed as inactive. Valid time units are
+; {s, m, h}.
+; ws-pong-wait=5s
+
 ; Shortest backoff when reconnecting to persistent peers. Valid time units are
 ; {s, m, h}.
 ; minbackoff=1s