JournalReader is not an Iterator

JournalReader is not an Iterator

Iterator dictating a two method API, which is not quite appropriate, as
the writer could be manipulated between hasNext() and next() methods --
and next() has to perform validation again.

Introduce JournalReader.tryNext(), which returns a @Nullable Indexed --
either the next entry (as next()), or null (indicating !hasNext() case).

JIRA: CONTROLLER-2106
Change-Id: Ie0338a9869ece8e6381ae719a29e97fd569b442f
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>