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/TJSONProtocol.cpp b/lib/cpp/src/protocol/TJSONProtocol.cpp
index b2ffeac..a7564d1 100644
--- a/lib/cpp/src/protocol/TJSONProtocol.cpp
+++ b/lib/cpp/src/protocol/TJSONProtocol.cpp
@@ -546,7 +546,7 @@
   return writeJSONArrayEnd();
 }
 
-uint32_t TJSONProtocol::writeStructBegin(const std::string& name) {
+uint32_t TJSONProtocol::writeStructBegin(const char* name) {
   return writeJSONObjectStart();
 }
 
@@ -554,7 +554,7 @@
   return writeJSONObjectEnd();
 }
 
-uint32_t TJSONProtocol::writeFieldBegin(const std::string& name,
+uint32_t TJSONProtocol::writeFieldBegin(const char* name,
                                         const TType fieldType,
                                         const int16_t fieldId) {
   uint32_t result = writeJSONInteger(fieldId);