MINOR: listener: define per-thr struct

MINOR: listener: define per-thr struct

Create a new structure li_per_thread. This is uses as an array in the
listener structure, with an entry allocated per thread. The new function
li_init_per_thr is responsible of the allocation.

For now, li_per_thread contains fields only useful for QUIC listeners.
As such, it is only allocated for QUIC listeners.