MINOR: tinfo: store the number of committed extra streams in the tgroup

MINOR: tinfo: store the number of committed extra streams in the tgroup

In order to be able to enforce global streams limitations, we'll first
have to be able to account how many streams we promised to serve via
frontend muxes. We'll always need to support at least one stream, which
is why here we're only counting extra streams beyond the first one. It
also has the benefit of leaving H1 out of this, and save it from updating
a variable. Also in order to avoid an important update cost, we're storing
this value per thread group. For now only H2 is implemented, but QUIC
should follow shortly and should only count bidirectional streams.