This commit changes `tsdb.mapFunc` to use `tsdb.MapInput` instead
of an iterator. This will make it easier and faster to pass blocks
of values from the new storage engine into the engine.
Instead of rounding up the points, sorting and then slicing, keep a
heap that allows us to quickly see if the point needs to be in the
set. This cuts a top query on a dataset of 8 million points from 35
seconds to 11 seconds.
Instead of using closures and 2 different sort routines, have an
interface compare method that makes it easy to switch directions
for comparisons.
Note that this changes the sort order of distinct to match that of
top. While it is a change, I don't think it will break any code. The
important thing for distinct is just that the ordering is absolute,
not what the order is.
Ben Johnson
Cory LaNou
Daniel Morsing
Daniel Morsing