Opening journal for reading later (lazy mode) doesn't create it.

This speeds up Knot startup when bootstrapping a lot of zones or
when no IXFR is used, as it doesn't have to create journal files
for each zone, only when required (received 'flush' command or
incoming incremental xfer).

Change-Id: I693f360c69172eaa6a51536de1c7efc3ff44aa27