rgw: make max_connections configurable in beast

rgw: make max_connections configurable in beast

Beast frontend currently accepts a hardcoded number of connections
that is defined by boost::asio::socket_base::max_connections. This
commit makes it configurable via a 'max_connections' config option
on rgw frontend.

Fixes: https://tracker.ceph.com/issues/43952
Signed-off-by: Tiago Pasqualini <tiago.pasqualini@canonical.com>
(cherry picked from commit d6dada5bcb356abaef8d9237ceca8f42d4fcfb74)

Conflicts:
        src/rgw/rgw_asio_frontend.cc
- trivial resolution