memcached: close socket at the end of request handling
memcached: close socket at the end of request handling

Fixes #64

scylladb/seastar,syuu1228/seastar,sjperkins/seastar,cloudius-systems/seastar,avikivity/seastar,scylladb/scylla-seastar,sjperkins/seastar,cloudius-systems/seastar,dreamsxin/seastar,syuu1228/seastar,dreamsxin/seastar,avikivity/seastar,cloudius-systems/seastar,scylladb/scylla-seastar,scylladb/seastar,scylladb/scylla-seastar,syuu1228/seastar,dreamsxin/seastar,kjniemi/seastar,scylladb/seastar,kjniemi/seastar,sjperkins/seastar,avikivity/seastar,kjniemi/seastar