Use shorten index key for hash-index

Summary:
I was wrong about the "index builder", right now since we create index
by scanning both whole table and index, there is not need to preserve
the whole key as the index key.

I switch back to original way index which is both space efficient and
able to supprot in-fly construction of hash index.

IN this patch, I made minimal change since I'm not sure if we still need
the "pluggable index builder", under current circumstance it is of no use
and kind of over-engineered. But I'm not sure if we can still exploit its
usefulness in the future; otherwise I think I can just burn them with great
vengeance.

Test Plan: unit tests

Reviewers: sdong, haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17745