Fix StreamIdGenerator performance (see comments inside)

[KowalczykBartek]

see comment

he c++ clients often have ~30K pending requests to backends - no big deal.

You can have code to check if session has been released, but you can also just explicitly document it.

Say, you can at-most be 65K requests away. It will fail otherwise.

we can have a roaring bitmap since this sessions have really nice properties of effectively monotinic increasing sequences mostly that tells the user if the request id is inuse.

IMO - we can defer that as long as we document it. it is rare - though has happened - on failure - for one client to have 65K outstanding reqs.