Fix valgrind issues

Summary:
NewFixedPrefixTransform is leaked in default options. Broken by b47812fba6

Also included in the diff some code cleanup

Test Plan:
valgrind env_test
also make check

Reviewers: haobo, danguo, yhchiang

Reviewed By: danguo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D17211

include/rocksdb/compaction_filter.h

@@ -139,6 +139,7 @@ class DefaultCompactionFilterFactory : public CompactionFilterFactory {
 //
 class CompactionFilterFactoryV2 {
  public:
+  // NOTE: CompactionFilterFactoryV2 will not delete prefix_extractor
   explicit CompactionFilterFactoryV2(const SliceTransform* prefix_extractor)
     : prefix_extractor_(prefix_extractor) { }
 
@@ -169,9 +170,8 @@ class CompactionFilterFactoryV2 {
 // return any filter
 class DefaultCompactionFilterFactoryV2 : public CompactionFilterFactoryV2 {
  public:
-  explicit DefaultCompactionFilterFactoryV2(
-    const SliceTransform* prefix_extractor)
-      : CompactionFilterFactoryV2(prefix_extractor) { }
+  explicit DefaultCompactionFilterFactoryV2()
+      : CompactionFilterFactoryV2(nullptr) { }
 
   virtual std::unique_ptr<CompactionFilterV2>
   CreateCompactionFilterV2(

include/rocksdb/options.h

@@ -126,6 +126,8 @@ struct Options {
 
   // Version TWO of the compaction_filter_factory
   // It supports rolling compaction
+  //
+  // Default: a factory that doesn't provide any object
   std::shared_ptr<CompactionFilterFactoryV2> compaction_filter_factory_v2;
 
   // If true, the database will be created if it is missing.