[rocksdb] new CompactionFilterV2 API

Summary:
This diff adds a new CompactionFilterV2 API that roll up the
decisions of kv pairs during compactions. These kv pairs must share the
same key prefix. They are buffered inside the db.

    typedef std::vector<Slice> SliceVector;
    virtual std::vector<bool> Filter(int level,
                                 const SliceVector& keys,
                                 const SliceVector& existing_values,
                                 std::vector<std::string>* new_values,
                                 std::vector<bool>* values_changed
                                 ) const = 0;

Application can override the Filter() function to operate
on the buffered kv pairs. More details in the inline documentation.

Test Plan:
make check. Added unit tests to make sure Keep, Delete,
Change all works.

Reviewers: haobo

CCs: leveldb

Differential Revision: https://reviews.facebook.net/D15087

include/rocksdb/env.h

@@ -214,7 +214,7 @@ class Env {
   virtual void StartThread(void (*function)(void* arg), void* arg) = 0;
 
   // Wait for all threads started by StartThread to terminate.
-  virtual void WaitForJoin() = 0;
+  virtual void WaitForJoin() {}
 
   // Get thread pool queue length for specific thrad pool.
   virtual unsigned int GetThreadPoolQueueLen(Priority pri = LOW) const {