BUILD: quic: fix overflow in global tune

BUILD: quic: fix overflow in global tune

A new global option was recently introduced to disable pacing. However,
the value used (1<<31) caused issue with some compiler as options field
used for storage is declared as int. Move pacing deactivation flag
outside into the newly defined quic_tune to fix this.

This should be backported up to 3.1 after a period of observation. Note
that it relied on the previous patch which defined new quic_tune type.