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