Disconnect JournalSegmentWriter from JournalWriter

Disconnect JournalSegmentWriter from JournalWriter

JournalSegmentWriter is really the internal API, so it should not have a
public API straightjacket. Disconnect the implementation, making method
properly package-private.

This also makes it very clear that JournalWriter is not something users
should close() -- because doing so would wreck internal machinery rather
thoroughly, rendering the segment writer inoperable -- and that should
only be possible via JournalSegment's lifecycle.

JIRA: CONTROLLER-2098
Change-Id: Ia18b0dc640cba11689bb36cf338c8cef3ae4e6f8
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 45661c71a7e3d762ceb7ac12ced3c383ba65ac76)