GEOMESA-139 Remove inconsistent naming of temporal and geometry attributes in index

GEOMESA-139 Remove inconsistent naming of temporal and geometry attributes in index

Standardize on using geom/dtg for index attributes:
geomesa_index_geometry   --> geom
geomesa_index_start_time --> dtg
and
geomesa_index_end_time   --> dtg_end_time
Is there an advantage to not having `import geomesa.core._` here?