MINOR: quic: ensure quic_conn is never removed on thread affinity rebind

MINOR: quic: ensure quic_conn is never removed on thread affinity rebind

On accept, quic_conn instance is migrated from its original thread to a
new one. This operation is conducted in two steps, on the original than
the new thread instance. During the interval, quic_conn is artificially
rendered inactive. It must never be accessed nor removed until migration
is completed via qc_finalize_affinity_rebind(). This new BUG_ON() will
enforce that removal is never conducted until migration is completed.