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

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

In 2d3a2108979adca6b7632e2d59c10e4b131e8bf4, 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.