commitlog: move segment_manager constructor outside the class definition
commitlog: move segment_manager constructor outside the class definition

We'll do that so we can, in following patches, use static members from
the segment. Those are not defined at this point.

Signed-off-by: Glauber Costa <7ba9037b0dd006f9408bd45b94707becd5cd4e87@scylladb.com>

scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,raphaelsc/scylla,raphaelsc/scylla,avikivity/scylla,duarten/scylla,raphaelsc/scylla,duarten/scylla