Introduce atomix.storage.journal.JournalSerdes

Introduce atomix.storage.journal.JournalSerdes

The interfaces are tightly coupled with namespace implementation, which
in turn is tightly coupled to Kryo.

As a first step to disconnect these three, introduce JournalSerdes,
which acts as the public API implemented by Namespace. This allows us to
hide atomix.utils.serializer package at least from OSGi.

Change-Id: I9668ba2eb3e0a58d7ed27ee09ec1bb3cee069eb4
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>