shared_promise: Add move assignment operator
shared_promise: Add move assignment operator

Message-Id: <a6b47be184df722e9eacb432109971dfec8e0db2@scylladb.com>

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