bug: heartbeats reach max substreams

[erhant]

Is it possible for heartbeats to reach max substreams before timeouts?

Let interval be h and timeout be t with max streams s.
We will have t / h heartbeats before we start to get timeouts, which means it will open that many substreams.

So, it must be that s > t / h (for the worst case scenario of all heartbeats timing out).

While this is 100 > 512 / 60 = ~8 right now, some people seem to get this max substream error still.