thread local pointer storage

Summary:
This is not a generic thread local implementation in the sense that it
only takes pointer. But it does support multiple instances per thread
and lets user plugin function to perform cleanup when thread exits or an
instance gets destroyed.

Test Plan: unit test for now

Reviewers: haobo, igor, sdong, dhruba

Reviewed By: igor

CC: leveldb, kailiu

Differential Revision: https://reviews.facebook.net/D16131

include/rocksdb/env.h

@@ -205,6 +205,9 @@ class Env {
   // When "function(arg)" returns, the thread will be destroyed.
   virtual void StartThread(void (*function)(void* arg), void* arg) = 0;
 
+  // Wait for all threads started by StartThread to terminate.
+  virtual void WaitForJoin() = 0;
+
   // *path is set to a temporary directory that can be used for testing. It may
   // or many not have just been created. The directory may or may not differ
   // between runs of the same process, but subsequent calls will return the
@@ -634,6 +637,7 @@ class EnvWrapper : public Env {
   void StartThread(void (*f)(void*), void* a) {
     return target_->StartThread(f, a);
   }
+  void WaitForJoin() { return target_->WaitForJoin(); }
   virtual Status GetTestDirectory(std::string* path) {
     return target_->GetTestDirectory(path);
   }