SegmentNodeStoreService vs. SegmentNodeStoreFactory

[Francesco Mari]

After reviewing and applying the patch for OAK-4978, I found myself
wondering if we need to make a distinction between
SegmentNodeStoreService and SegmentNodeStoreFactory. IIUC, the only
difference between the two seems to be the presence of a "role"
configuration property. The SegmentNodeStoreService seems to be a
special case of SegmentNodeStoreFactory where the "role" property is
unspecified. Should we remove SegmentNodeStoreService in favour of
SegmentNodeStoreFactory?