BUG/MINOR: quic: fix zero value of tune.quic.sec.stateless-reset-rate

BUG/MINOR: quic: fix zero value of tune.quic.sec.stateless-reset-rate

quic_stateless_reset_rate_init() split <quic_tune.sec_stateless_reset_rate>
across threads as (rate + tid) / nbthread, which can round down to 0 when
<rate> is lower than <nbthread>. send_stateless_reset() reads a 0 value as
"no limit", so a low <rate> silently disabled rate limiting on some
threads instead of restricting it further.

Fix it by giving every thread the same value, rounded up so it can never
reach 0: (rate + nbthread - 1) / nbthread. Updated the doc accordingly.

This bug was introduced by commit 297321e3e ("MINOR: quic: add
tune.quic.sec.stateless-reset-rate"), only present in this dev branch,
so no backport is needed.