C++: Make write{Struct,Field}Begin take "name" as a const char*.

Summary:
These methods previously took name as a const string&.  While that way
is more idiomatic, it requires a temporary string to be constructed
when we pass a string literal (which is always).  This was significantly
slowing down the serialization of field-heavy structures.  This change
will break ABI compatibility, but the serialization speed boost with
no external API changes is too important to pass up.

Reviewed By: mcslee, aditya

Test Plan: make check

TracCamp Project: Thrift

Revert Plan: ok


git-svn-id: https://svn.apache.org/repos/asf/incubator/thrift/trunk@665672 13f79535-47bb-0310-9956-ffa450edef68
diff --git a/lib/cpp/src/protocol/TDenseProtocol.h b/lib/cpp/src/protocol/TDenseProtocol.h
index 713b57a..5dd5a8c 100644
--- a/lib/cpp/src/protocol/TDenseProtocol.h
+++ b/lib/cpp/src/protocol/TDenseProtocol.h
@@ -84,11 +84,11 @@
   virtual uint32_t writeMessageEnd();
 
 
-  virtual uint32_t writeStructBegin(const std::string& name);
+  virtual uint32_t writeStructBegin(const char* name);
 
   virtual uint32_t writeStructEnd();
 
-  virtual uint32_t writeFieldBegin(const std::string& name,
+  virtual uint32_t writeFieldBegin(const char* name,
                                    const TType fieldType,
                                    const int16_t fieldId);