future: Use placement new instead of operator= in set_urgent_state
future: Use placement new instead of operator= in set_urgent_state

We know that the state can only be set once, so no point in destroying
a state that is in the future state before overwriting it.

This reduces the text size in scylla by 1.2%, from 38008468 to
37550772 bytes.

Signed-off-by: Rafael Ávila de Espíndola <1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>
Message-Id: <20200326162846.11485-2-1975039f9046bf513b77c0c5639a0efc25f00e7f@scylladb.com>

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