BUG/MINOR: mux_quic: fix stream elasticity dec on qcm_init() failure

BUG/MINOR: mux_quic: fix stream elasticity dec on qcm_init() failure

On qcm_init() early failure, QCC is freed via qcc_release(). This
function always decrements reserved streams from the thread group pool
when stream elasticity is active for frontend connections.

This causes an issue as qcm_init() may fail before accounting the
committed streams: this is the case for <pacing_task> allocation
failure. Thread group counter is thus invalid after the extra decrement
and may cause an underflow and wrap.

Fix this by moving pacing task allocation later in qcm_init() so that
streams accounting is now performed before any failure path.

This must be backported up to 3.4.