[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

Author: gdhdanny

HISTORY.md

@@ -15,11 +15,13 @@
 * Added Env::GetThreadPoolQueueLen(), which returns the waiting queue length of thread pools
 * Added a command "checkconsistency" in ldb tool, which checks
   if file system state matches DB state (file existence and file sizes)
+* CompactionFilter::Context is now CompactionFilterContext. It is shared by CompactionFilter and CompactionFilterV2
 
 ### New Features
 * If we find one truncated record at the end of the MANIFEST or WAL files,
   we will ignore it. We assume that writers of these records were interrupted
   and that we can safely ignore it.
+* Now compaction filter has a V2 interface. It buffers the kv-pairs sharing the same key prefix, process them in batches, and return the batched results back to DB.
 
 ## 2.7.0 (01/28/2014)