Move JournalWriter.getLastIndex()

Move JournalWriter.getLastIndex()

Last written index is a property of a particular Journal, not of a
writer -- and now that we maintain this in the index, we can make
shortcuts.

This also removes a source of confusion, as we have two methods taking
a 'long index' and performing some writer adjustments:
- reset(long) is equivalent of setNextIndex()
- truncate(long) is equivalent of setLastIndex()

Change-Id: I1bc4b5d1b3052c2b35808b8ec4ea2d88dcfca593
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>