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