Disconnect JournalSegmentReader from JournalReader

JournalSegmentReader is the only used by SegmentedJournalReader, which
is the actual implementation of JournalReader.

There is really no point in JournalSegmentReader to conform to the
public API, as not all elements are needed.

This disconnects JournalSegmentReader, eliminating getFirstIndex(), as
that is simply not called -- and was not conforming to the API anyway.

JIRA: CONTROLLER-2098
Change-Id: Icd802ee07207d4f6787e3bdb71e36e553f2010c3
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>