JournalIndex.truncate() should return last entry

The only time we are calling truncate(), we are following up with a seek
to the appropriate entry.

Return the last indexed position from truncate(), so we can do better
than a linear search.

JIRA: CONTROLLER-2109
Change-Id: I701b0eefbf1f4d83f74ba95b93c41407289d95cb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>