Use memory-mapped segmented journal by default

Segmented file implementation requires a buffer double the size
of the file's maximum size. This ends up being allocated on-healp, which
is wasteful from effeciency perspective. Switch default configuration
so that it uses memory-mapped files instead.

JIRA: CONTROLLER-1954
Change-Id: Icad9ef74c50467323e31567828a949c0cae52a9e
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>