Synchronize SegmentedJournal.open()

SpotBugs is pointing out a problem with synchronization to
currentSegment. This is a false positive, as we only call this method
from the constructor, but slap a synchronized keyword on it anyway.

JIRA: CONTROLLER-2071
Change-Id: Iaf3a13c4122a3321b615eb5629327494165c0bed
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>