optimizations from profiling build_hyracks

@anbangx @Nan-Zhang @JavierJia @Elmira88 

Some simple changes to our code after looking at the profiles of BUILD_HYRACKS.  the `enum.values()` function was apparently still doing a lot of work (or was being called so many times as to make it seem that way).  I swapped it with a static final array of the same name.

6f40f96 will cause more GC overhead but it's negligible compared to the deepcopy cost.  Running timings now.