BUG/MEDIUM: ssl: Fix unprotected 'ssl_sock_choose_sni_ctx' calls

BUG/MEDIUM: ssl: Fix unprotected 'ssl_sock_choose_sni_ctx' calls

The 'ssl_sock_choose_sni_ctx' function accesses the sni tree and it
mostly covered by a read/write lock which was not taken around two of
its uses. One of them could be reached by using the SSL-related CLI
commands and the other one when the generate-cert mechanism is used. So
even if it's in a not that common use case, a race could still happen
and end up crashing.

Note that this requires that the read lock is properly released
before calling ssl_sock_generate_certificate() so as to avoid a
possible read-lock recursion (it was verified that this is safe).

This patch should be backported up to 3.0.

Reported-by: Claude (ANT-2026-VFR5JH3R)