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
Renamed InfoLogLevel::DEBUG to InfoLogLevel::DEBUG_LEVEL

Summary: XCode for some reason injects `#define DEBUG 1` into our code, which makes compile fail because we use `DEBUG` keyword for other stuff. This diff fixes the issue by renaming `DEBUG` to `DEBUG_LEVEL`.

Test Plan: compiles

Reviewers: dhruba, haobo, sdong, yhchiang, ljin

Reviewed By: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17709
[JNI] Add readwhilewriting to Java db_bench

Summary: Add readwhilewriting to Java db_bench

Test Plan:
make jni
make jdb_bench

Reviewers: haobo, ankgup87, sdong, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17619
Fix some InfoLogLevels
Temporarily disable a test case in db_test

Summary:

Root cause is still under investigation. Just Disable the troubling use case for now.
No shadow in public headers
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