Made merge_oprator a shared_ptr; and added TTL unit tests

Test Plan:
- make all check;
- make release;
- make stringappend_test; ./stringappend_test

Reviewers: haobo, emayanke

Reviewed By: haobo

CC: leveldb, kailiu

Differential Revision: https://reviews.facebook.net/D12381

include/leveldb/options.h

@@ -85,7 +85,7 @@ struct Options {
   // for the first time. It's necessary to specify a merge operator when
   // openning the DB in this case.
   // Default: nullptr
-  const MergeOperator* merge_operator;
+  shared_ptr<MergeOperator> merge_operator;
 
   // The client must provide compaction_filter_factory if it requires a new
   // compaction filter to be used for different compaction processes