Push down limits into Kudu

The progress made in KUDU-16 introduced a way to impose limits on Kudu scanners, potentially reducing the number of RPCs sent per scan, CPU used for scanner evaluation on the tablet server, etc. It would be nice if Impala could make use of this new behavior.

I put up an admittedly clunky patch that implements limits on a per-token basis. I'm no Impala expert, so there may be an API missing in Kudu that would make Impala's life easier in implementing this. Maybe it's as easy as adjusting the limit after re-hydrating the token into a scanner.