Fix overflow in `awaiting_pong_timer...` with too many peers

Fix overflow in `awaiting_pong_timer...` with too many peers

If we have more than
127 / `MAX_BUFFER_DRAIN_TICK_INTERVALS_PER_PEER` (31) peers,
`awaiting_pong_timer_tick_intervals` can overflow before we hit
the limit. This isn't super harmful, we'll still disconnect peers
as long as they don't send *any* messages between two pings, but it
does cause us to not disconnect peers which are extremely slow in
responding to messages, e.g. because they are overloaded.