[C++]: add wrapAndApplyHeader for ease of use.

Author: tmontgomery

sbe-tool/src/main/java/uk/co/real_logic/sbe/generation/cpp/CppGenerator.java

@@ -1693,6 +1693,19 @@ private CharSequence generateMessageFlyweightCode(final String className, final
  " reset(buffer, offset, bufferLength, sbeBlockLength(), sbeSchemaVersion());\n" +
  " return *this;\n" +
  " }\n\n" +
+ " %10$s &wrapAndApplyHeader( " +
+ "char *buffer, const std::uint64_t offset, const std::uint64_t bufferLength)\n" +
+ " {\n" +
+ " MessageHeader hdr(buffer + offset, bufferLength, sbeSchemaVersion());\n\n" +
+ " hdr\n" +
+ " .blockLength(sbeBlockLength())\n" +
+ " .templateId(sbeTemplateId())\n" +
+ " .schemaId(sbeSchemaId())\n" +
+ " .version(sbeSchemaVersion());\n\n" +
+ " reset(buffer + offset + MessageHeader::encodedLength(), 0, bufferLength, sbeBlockLength(), " +
+ "sbeSchemaVersion());\n" +
+ " return *this;\n" +
+ " }\n\n" +
  " %10$s &wrapForDecode(\n" +
  " char *buffer, const std::uint64_t offset, const std::uint64_t actingBlockLength,\n" +
  " const std::uint64_t actingVersion, const std::uint64_t bufferLength)\n" +