Added collect_merge_max (not working)
Finished collect_merge_max
Optimized map_key_values
Fixed collect_merge_max bug for strings & timestamps
Removed unused code in udf.collect.AbstractCollectMergeUDAF
Made collect_merge_max more null-safe
Optimized udf.collect.CollectMergeMaxUDAF to produce less temporary boxed primitives
Merge pull request #66 from leventov/master

collect_merge skeleton; collect_merge_max; optimized map_key_values a bit