last() would always return the last output of the iterator (which isn't
necessarily the last time value due to how the merge iterator works) and
first() would always return the first output of the iterator (wrong for
the same reason).
Now the time is kept by the reduce function and the times are wiped as
part of the reduce iterator after the value has been found.
It matches more in functionality to the functions in call_iterator.go
than iterator.go. iterator.go mostly has base iterators and
call_iterator.go has iterators related to functional calls, which is
the only time integerReduceSliceFloatIterator is used.
Also fixes the `first()` and `last()` calls to do the same thing as
`min()` and `max()` by returning the time corresponding to the start of
the interval rather than the point's real time.
This does not implement the time selector, but everything else is
implemented. Unfortunately, there are no tests for bottom() in the old
query engine, so only top() is properly tested.
Ben Johnson
Jonathan A. Sternberg