Enable log info with different levels.

Summary:
* Now each Log related function has a variant that takes an additional
  argument indicating its log level, which is one of the following:
 - DEBUG, INFO, WARN, ERROR, FATAL.

* To ensure backward-compatibility, old version Log functions are kept
  unchanged.

* Logger now has a member variable indicating its log level.  Any incoming
  Log request which log level is lower than Logger's log level will not
  be output.

* The output of the newer version Log will be prefixed by its log level.

Test Plan:
Add a LogType test in auto_roll_logger_test.cc

 = Sample log output =
    2014/02/11-00:03:07.683895 7feded179840 [DEBUG] this is the message to be written to the log file!!
    2014/02/11-00:03:07.683898 7feded179840 [INFO] this is the message to be written to the log file!!
    2014/02/11-00:03:07.683900 7feded179840 [WARN] this is the message to be written to the log file!!
    2014/02/11-00:03:07.683903 7feded179840 [ERROR] this is the message to be written to the log file!!
    2014/02/11-00:03:07.683906 7feded179840 [FATAL] this is the message to be written to the log file!!

Reviewers: dhruba, xjin, kailiu

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16071
PlainTable::Next() should pass the error message from ReadKey()

Summary:
PlainTable::Next() should pass the error message from ReadKey(). Now it would return a wrong error message.
Also improve the messages of status when failing to read

Test Plan: make all check

Reviewers: ljin, kailiu, haobo

Reviewed By: kailiu

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16365
Fix inconsistent code format

Summary:
Found some function follows camel style. When naming funciton, we have two styles:

Trivially expose internal data in readonly mode: `all_lower_case()`
Regular function: `CapitalizeFirstLetter()`

I renames these functions.

Test Plan: make -j32

Reviewers: haobo, sdong, dhruba, igor

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16383
Make sure logger is safely released in `InfoLogLevel`

Summary: fix the memory leak that was captured by jenkin build.

Test Plan: ran the valgrind test locally

Reviewers: yhchiang

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16389
cache SuperVersion in thread local storage to avoid mutex lock

Summary: as title

Test Plan:
asan_check
will post results later

Reviewers: haobo, igor, dhruba, sdong

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D16257