file: Toss append-challenged file constructor
file: Toss append-challenged file constructor

Some fields can be initialized in the initializer list part.
Next patch makes real use of it.

Signed-off-by: Pavel Emelyanov <c9a32589e048e044184536f7ac71ef92fe82df3e@scylladb.com>

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