PlainTableBuilder::Add() to use local char array instead of reused std::string as tmp buffer

Summary: Our profile shows that in one of the applications, 5% of the CPU costs of PlainTableBuilder::Add() are spent on std::string stacks. By this simple change, we avoid this global reusable string. Also, we avoid another call of file appending, which probably gives another 2%.

Test Plan: make all check

Reviewers: haobo, ljin

Reviewed By: haobo

CC: igor, yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D17601
Make the block-based table's index pluggable

Summary:
This patch introduced a new table options that allows us to make
block-based table's index pluggable.

To support that new features:

* Code has been refacotred to be more flexible and supports this option well.
* More documentation is added for the existing obsecure functionalities.
* Big surgeon on DataBlockReader(), where the logic was really convoluted.
* Other small code cleanups.

The pluggablility will mostly affect development of internal modules
and won't change frequently, as a result I intentionally avoid
heavy-weight patterns (like factory) and try to make it simple.

Test Plan: make all check

Reviewers: haobo, sdong

Reviewed By: sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16395

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