memcached: Fix use after free in shutdown
memcached: Fix use after free in shutdown

Before this patch memcached could hit a user after free during
shutdown since nothing was waiting for the server to shutdown before
continuing.

With this patch there is no user after free or memory leak. This fixes
the memcached test in a debug build.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20191231194721.41240-1-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

cloudius-systems/seastar,avikivity/seastar,dreamsxin/seastar,dreamsxin/seastar,cloudius-systems/seastar,syuu1228/seastar,scylladb/seastar,dreamsxin/seastar,cloudius-systems/seastar,scylladb/seastar,avikivity/seastar,syuu1228/seastar,syuu1228/seastar,scylladb/seastar,avikivity/seastar