res_pjsip_config_wizard: Fix concurrent Named ACL reloads

res_pjsip_config_wizard: Fix concurrent Named ACL reloads

Named ACL change notifications reloaded PJSIP sorcery directly from the
Stasis callback. Under concurrent ACL, core, and config wizard reloads, that
could overlap with config wizard object type observers that compare, prune,
destroy, and replace the per-object-type last_config cache.

Route the ACL-triggered reload through the PJSIP servant path and track when
a Named ACL reload is pending or running so the observer still bypasses the
unchanged-file optimization. Also serialize the observer cache update to
prevent overlapping reload paths from mutating last_config at the same time.

UserNote: Fixes a PJSIP config wizard reload hang during overlapping reloads.