[RocksDB] [Performance Branch] Revert previous patch.

Summary: The previous patch is wrong. rep_.resize(kHeader) just resets the header portion to zero, and should not cause a re-allocation if g++ does it right. I will go ahead and revert it.

Test Plan: make check

Reviewers: dhruba, sdong

CC: leveldb

Differential Revision: https://reviews.facebook.net/D14793

db/write_batch.cc

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