Simplify FileChannelJournalSegmentWriter position tracking
This patch makes the obvious switch in state tracking: memory buffer
does not hold anything of value w.r.t. overall state, so there is no
point in marking/resetting and maintaining memory.position() beyond
what we need for reading from it.
Local 'position' acts now as the authoritative source of where we want
to go and we propagate it to channel position when we validate an entry.
JIRA: CONTROLLER-2095
Change-Id: Iab150a3c7348c714170223b397167cd4bb87f087
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>