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
Yes, the use of `SF_PROPERTY_START_TIME` for the key and the dtg attribute's name in different parts of the code had confused me. The above cannot be changed, but some explicit references to  `SF_PROPERTY_START_TIME` can be changed/removed. And queries should not refer to it either.