[RocksDB] [Performance Branch] Minor fix, Remove string resize from WriteBatch::Clear

Summary: tmp_batch_ will get re-allocated for every merged write batch because of the existing resize in WriteBatch::Clear. Note that in DBImpl::BuildBatchGroup, we have a hard coded upper limit of batch size 1<<20 = 1MB already.

Test Plan: make check

Reviewers: dhruba, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14787

db/write_batch.cc

@@ -58,7 +58,6 @@ bool WriteBatch::Handler::Continue() {
 
 void WriteBatch::Clear() {
   rep_.clear();
-  rep_.resize(kHeader);
 }
 
 int WriteBatch::Count() const {