peer+lnd: add new CLI option to control if we D/C on slow pongs

In this commit, we add a new CLI option to control if we D/C on slow
pongs or not. Due to the existence of head-of-the-line blocking at
various levels of abstraction (app buffer, slow processing, TCP kernel
buffers, etc), if there's a flurry of gossip messages (eg: 1K channel
updates), then even with a reasonable processing latency, a peer may
still not read our ping in time.

To give users another option, we add a flag that allows users to disable
this behavior. The default remains.

sample-lnd.conf

@@ -569,6 +569,11 @@
 ; The number of restricted slots the server will allocate for peers.
 ; num-restricted-slots=30
 
+; If true, a peer will *not* be disconnected if a pong is not received in time
+; or is mismatched. Defaults to false, meaning peers *will* be disconnected on
+; pong failure.
+; no-disconnect-on-pong-failure=false
+
 [fee]
 
 ; Optional URL for external fee estimation. If no URL is specified, the method