Better port::Mutex::AssertHeld() and AssertNotHeld()

Summary:
Using ThreadLocalPtr as a flag to determine if a mutex is locked or not enables us to implement AssertNotHeld(). It also makes AssertHeld() actually correct.

I had to remove port::Mutex as a dependency for util/thread_local.h, but that's fine since we can just use std::mutex :)

Test Plan: make check

Reviewers: ljin, dhruba, haobo, sdong, yhchiang

Reviewed By: ljin

CC: leveldb

Differential Revision: https://reviews.facebook.net/D18171
Revert "Better port::Mutex::AssertHeld() and AssertNotHeld()"

This reverts commit ddafceb6c2ecb83b7bdf6711ea1c30d97aeb3b8f.
PlainTableReader to expose index size to users

Summary:
This is a temp solution to expose index sizes to users from PlainTableReader before we persistent them to files.
In this patch, the memory consumption of indexes used by PlainTableReader will be reported as two user defined properties, so that users can monitor them.

Test Plan:
Add a unit test.
make all check`

Reviewers: haobo, ljin

Reviewed By: haobo

CC: nkg-, yhchiang, igor, ljin, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D18195