Review for hybrid-hash-grouper, and updates on aggregator interface

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Purpose of code changes on this branch:
hyracks_external_gby_fix contains the latest performance improvement on the 
external hash group operator, and also new grouping algorithm using hybrid hash.

When reviewing my code changes, please focus on:
r1334 and r1335 include only fixes about external grouper; r1336 contains codes 
for hybrid-hash-grouper. In case that hybrid-hash-grouper will not be merged to 
trunk now, we can do the merge until r1335.

@yingyi I have to have some changes in the aggregator interface to include new 
methods required by the new code. I have implemented the missing methods in the 
affected algrbrick code, however I am not sure whether the modification is 
correct. Please have a look at them.

@vinayak the hybrid-hash-grouper needs to be reviewed, so that the 
implementation is on the same page as our discussion before.

After the review, I'll merge this branch into:
/trunk

Original issue reported on code.google.com by jarod...@gmail.com on 3 Apr 2012 at 7:22

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Original comment by buyingyi@gmail.com on 3 Apr 2012 at 8:20