[RocksDB] [Performance Bug] MemTable::Get Slow

Summary:
The merge operator diff introduced a performance problem in MemTable::Get.
An exit condition is missed when the current key does not match the user key.
This could lead to full memtable scan if the user key is not found.

Test Plan: make check; db_bench

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10851

db/memtable.cc

@@ -185,6 +185,9 @@ bool MemTable::Get(const LookupKey& key, std::string* value, Status* s,
           return true;
         }
       }
+    } else {
+      // exit loop if user key does not match
+      break;
     }
   }