Move entry tracking to SegmentedJournalReader

Move entry tracking to SegmentedJournalReader

We have duplicated state tracking just because JournalSegmentReader used
to provide the same API as SegmentedJournalReader.

Rehost tracking the next index and current entry into
SegmentedJournalReader, making it more obvious as to what is going on.

This allows Indexed to enforce the entry being non-null, as we do not
have to use a fake entry anymore.

JIRA: CONTROLLER-2109
Change-Id: I9ebd50fde94d695bb62315bba1b6adb1235b37f1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>