Merge operator fixes part 1.

Summary:
-Added null checks and revisions to DBIter::MergeValuesNewToOld()
-Added DBIter test to stringappend_test
-Major fix with Merge and TTL
More plans for fixes later.

Test Plan:
-make clean; make stringappend_test -j 32; ./stringappend_test
-make all check;

Reviewers: haobo, emayanke, vamsi, dhruba

Reviewed By: haobo

CC: leveldb

Differential Revision: https://reviews.facebook.net/D12315

include/utilities/stackable_db.h

@@ -60,7 +60,7 @@ class StackableDB : public DB {
                            const Slice& key,
                            std::string* value,
                            bool* value_found = nullptr) override {
-    return KeyMayExist(options, key, value, value_found);
+    return sdb_->KeyMayExist(options, key, value, value_found);
   }
 
   virtual Status Delete(const WriteOptions& wopts, const Slice& key) override {