BUG/MEDIUM: quic: quic_cc_conn ->cntrs counters unreachable

BUG/MEDIUM: quic: quic_cc_conn ->cntrs counters unreachable

This bug arrived with this commit in 2.9-dev3:

    MEDIUM: quic: Allow the quic_conn memory to be asap released.

When sending packets from quic_cc_conn_io_cb(), e.g. when the quic_conn
object has been released and replaced by a lighter one (quic_cc_conn),
some counters may have to be incremented. But they were not reachable
because not shared between quic_conn and quic_cc_conn struct.

To fix this, one has only to move the ->cntrs counters from quic_conn
to QUIC_CONN_COMMON struct which is shared between both quic_cc_conn

Thank you to Tristan for having reported this in GH #2247.

No need to backport.