atomic-storage: remove type dependency at segment level I/O

atomic-storage: remove type dependency at segment level I/O

Due to object de- and serialization was served by segment
I/O artifacts it required these artifacts holding serializer
instance and being dependent on its type (generics).

Moving serialization to upper level (journal) allows segment
reader/writer operate on byte level only with no extra data
conversions. Serializer interface simplified for further
isolation and removal of Kryo serializer.

JIRA: CONTROLLER-2115
Change-Id: I8f6bac3192a0f38b627150be4c8ea128f1e233e5
Signed-off-by: Ruslan Kashapov <ruslan.kashapov@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>