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

util/thread_local.h

@@ -10,13 +10,12 @@
 #pragma once
 
 #include <atomic>
+#include <mutex>
 #include <memory>
 #include <unordered_map>
 #include <vector>
 
 #include "util/autovector.h"
-#include "port/port_posix.h"
-#include "util/thread_local.h"
 
 namespace rocksdb {
 
@@ -153,7 +152,7 @@ class ThreadLocalPtr {
 
     // protect inst, next_instance_id_, free_instance_ids_, head_,
     // ThreadData.entries
-    static port::Mutex mutex_;
+    static std::mutex mutex_;
 #if !defined(OS_MACOSX)
     // Thread local storage
     static __thread ThreadData* tls_;