reactor: Comment why _backend is assigned in constructor body
reactor: Comment why _backend is assigned in constructor body

As per Rafael's request.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>
Message-Id: <20200113173910.11167-1-c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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