Enable hash index for block-based table

Summary: Based on previous patches, this diff eventually provides the end-to-end mechanism for users to specify the hash-index.

Test Plan: Wrote several new unit tests.

Reviewers: sdong, haobo, dhruba

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16539

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

Conflicts:
	table/block_based_table_reader.cc
	table/block_based_table_reader.h