Reduce the number of timer ticks a peer is allowed to take

In 2d3a210, we increased the
default ping timer in `lightning-background-processor` to ten
seconds from five. However, we didn't change the timer count at
which we disconnect peers if they're not responding, which we
likely should have done. We do so here, as well as update the
documentation for `PeerManager::timer_tick_occurred` to suggest
always ticking the timer every ten seconds instead of five.

Author: TheBlueMatt

lightning/src/ln/peer_handler.rs

@@ -307,12 +307,12 @@ const OUTBOUND_BUFFER_LIMIT_DROP_GOSSIP: usize = OUTBOUND_BUFFER_LIMIT_READ_PAUS
 /// Thus, to avoid needlessly disconnecting a peer, we allow a peer to take this many timer ticks
 /// per connected peer to respond to a ping, as long as they send us at least one message during
 /// each tick, ensuring we aren't actually just disconnected.
-/// With a timer tick interval of five seconds, this translates to about 30 seconds per connected
+/// With a timer tick interval of ten seconds, this translates to about 40 seconds per connected
 /// peer.
 ///
 /// When we improve parallelism somewhat we should reduce this to e.g. this many timer ticks per
 /// two connected peers, assuming most LDK-running systems have at least two cores.
-const MAX_BUFFER_DRAIN_TICK_INTERVALS_PER_PEER: i8 = 6;
+const MAX_BUFFER_DRAIN_TICK_INTERVALS_PER_PEER: i8 = 4;
 
 /// This is the minimum number of messages we expect a peer to be able to handle within one timer
 /// tick. Once we have sent this many messages since the last ping, we send a ping right away to
@@ -1572,9 +1572,9 @@ impl<Descriptor: SocketDescriptor, CM: Deref, RM: Deref, L: Deref, CMH: Deref> P
 	/// Send pings to each peer and disconnect those which did not respond to the last round of
 	/// pings.
 	///
-	/// This may be called on any timescale you want, however, roughly once every five to ten
-	/// seconds is preferred. The call rate determines both how often we send a ping to our peers
-	/// and how much time they have to respond before we disconnect them.
+	/// This may be called on any timescale you want, however, roughly once every ten seconds is
+	/// preferred. The call rate determines both how often we send a ping to our peers and how much
+	/// time they have to respond before we disconnect them.
 	///
 	/// May call [`send_data`] on all [`SocketDescriptor`]s. Thus, be very careful with reentrancy
 	/// issues!