Feature Request: Currently open stream statistic

[splitice]

Is your feature request related to a problem? Please describe.

Related to #383 and #386

Describe the solution you'd like
It would be really handy to know the current number of opened streams on a session at the ngtcp2 level. This would make it easy to identify leaking streams (i.e ones lost in the Javascript client) but still open due to not (yet) exceeding the idle time.

This is supplementary to the current counters providing the total number of opened streams since session establishment.

Describe alternatives you've considered
Of course the user could wrap the QUIC system to provide tracking like this. However as this relates to finding a common possible developer error it might be best to provide it in the core. Further as the Native module can likely (confirm?) extract this from ngtcp2 or it's own data it can likely provide a more reliable source for this data.