PlainTableBuilder::Add() to use local char array instead of reused std::string as tmp buffer

Summary: Our profile shows that in one of the applications, 5% of the CPU costs of PlainTableBuilder::Add() are spent on std::string stacks. By this simple change, we avoid this global reusable string. Also, we avoid another call of file appending, which probably gives another 2%.

Test Plan: make all check

Reviewers: haobo, ljin

Reviewed By: haobo

CC: igor, yhchiang, dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D17601

table/plain_table_builder.h

@@ -69,9 +69,6 @@ private:
   const size_t user_key_len_;
   bool closed_ = false;  // Either Finish() or Abandon() has been called.
 
-  std::string key_size_str_;
-  std::string value_size_str_;
-
   bool IsFixedLength() const {
     return user_key_len_ > 0;
   }