Currently we were duplicating code for AS OF SYSTEM TIME in several
places. This replaces the code with using a method on
dbutil.Implementation.
As a consequence it's more useful to use a shorter name for
implementation - 'impl' should be sufficiently clear in the context.
Similarly, using AsOfSystemInterval and AsOfSystemTime to distinguish
between the two modes is useful and slightly shorter without causing
confusion.
Change-Id: Idefe55528efa758b6176591017b6572a8d443e3d