Output meta in sort reducer using multiple output

[at15]

currently, the meta for each partition is generated in the index building phase, however, if using mellowtech's bptree, it seems I need to traverse the data twice to get the min and max key, because iterator is needed for bulk loading, and bptree does not provide max and min key.

• [x] generate meta file when partition
• [ ] change meta mr job source, meta table should still build based on index result because it need the position for the index file

[at15]

oops, no, it seems to be impossible to generate the range, unless there is some before after method

[at15]

yep, the cleanup method is called when one reducer finished its job, so I can get the max and min key there.

[at15]

finished in ddcd677c96b6f826273eed983f6d2b02fba3b274