Always fill empty groups if the start time is specified.

Prior to this change, empty group filling worked between t1 and t2,
where t1 is the timestamp of the earliest point and t2 is the timestamp
of the last point. This patch change the behavior of the fill() to use
the query start and end time as t1 and t2, respectively. This only
happens if the user specified the start time of the query. Otherwise,
there's a potential of filling millions of millions of groups, since the
default start time of the query is really really early.