In 2d3a210897, 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.