MINOR: mux_quic: implement basic committed_extra_streams accounting

MINOR: mux_quic: implement basic committed_extra_streams accounting

Account QUIC frontend connections into committed_extra_streams when
stream elasticity setting is active. This is performed in QCC init and
release functions.

This patch has no impact on QUIC subsystem for now. Connections will
still allow a static number of concurrent streams based on
tune.quic.fe.stream.max-concurrent. However, this has a direct
repercussion on H2 subsystem, as a higher count of QUIC connections will
reduce the concurrent streams allowed there.