MINOR: quic: add tune.quic.sec.stateless-reset-rate

MINOR: quic: add tune.quic.sec.stateless-reset-rate

RFC 9000 10.3 allows sending a Stateless Reset for any packet that can't
be matched to a connection. A flood of garbage UDP datagrams can turn this
into an unwitting amplifier, wasting CPU and bandwidth on replies.

Add tune.quic.sec.stateless-reset-rate to cap the total number of
Stateless Reset packets sent per second across all threads (default:
1000). It is not scoped to the frontend side: a Stateless Reset can be
sent for any unmatched packet, regardless of which side received it.
Each thread keeps its own THREAD_LOCAL freq_ctr and gets its own share
of the total, computed once at thread startup: thread <tid> gets
(N + tid) / nbthread, so the shares always add up to exactly N (no
rounding overshoot.

Tested by flooding a listener with short-header packets carrying random
DCIDs: the number of replies received matches the configured rate rather
than the number of packets sent, with normal traffic unaffected when well
under the limit.