Fix Order By Desc

[corylanou]

Used the heap.Interface to be able to support a Reverse interface on TagSetCursor when it is order by DESC

fixes #4235.

[corylanou]

Current test output (broken as expected based on issue):

query:  select value from "power" ORDER BY time DESC
exp:    {"results":[{"series":[{"name":"power","columns":["time","value"],"values":[["2000-01-01T00:00:04Z",4],["2000-01-01T00:00:03Z",3],["2000-01-01T00:00:02Z",2],["2000-01-01T00:00:01Z",1]]}]}]}
actual: {"results":[{"series":[{"name":"power","columns":["time","value"],"values":[["2000-01-01T00:00:03Z",3],["2000-01-01T00:00:02Z",2],["2000-01-01T00:00:04Z",4],["2000-01-01T00:00:01Z",1]]}]}]}

[otoolep]

Great change, a lovely example of interfaces. +1 on green.

[otoolep]

Only thing you might prefer is that the "constructor" could be of the form newPointHeap(ascending bool).