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

utilities/merge_operators/string_append/stringappend2.cc

@@ -20,11 +20,12 @@ StringAppendTESTOperator::StringAppendTESTOperator(char delim_char)
 }
 
 // Implementation for the merge operation (concatenates two strings)
-bool StringAppendTESTOperator::Merge(const Slice& key,
-                                     const Slice* existing_value,
-                                     const std::deque<std::string>& operands,
-                                     std::string* new_value,
-                                     Logger* logger) const {
+bool StringAppendTESTOperator::FullMerge(
+    const Slice& key,
+    const Slice* existing_value,
+    const std::deque<std::string>& operands,
+    std::string* new_value,
+    Logger* logger) const {
 
   // Clear the *new_value for writing.
   assert(new_value);