shared_mutex: make lock methods noexcept
shared_mutex: make lock methods noexcept

Catch and handle allocation failures from circular_buffer::emplace_back.

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

syuu1228/seastar,syuu1228/seastar,scylladb/seastar,scylladb/seastar,avikivity/seastar,avikivity/seastar,syuu1228/seastar,avikivity/seastar,scylladb/seastar