GEOMESA-134: DTG field treatment in package object 'index' not safe

Wrap dtg variables in core.index in Options, since the temporal field in...

Also modify FilterToAccumulo to handle these changes.
@ronq I believe I ran into it on the BETWEEN query before too
@cne1x we should make the defaults "geom" and "dtg" instead of "geomesa_index_start_time" and "geomesa_index_geometry" if we go with a default.