[C++] Add an accessor for bufferLength on messages. Issue #479.

Author: mjpt777

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

@@ -1441,6 +1441,10 @@ private CharSequence generateMessageFlyweightCode(final String className, final
             "    {\n" +
             "        return m_buffer;\n" +
             "    }\n\n" +
+            "    std::uint64_t bufferLength() const SBE_NOEXCEPT\n" +
+            "    {\n" +
+            "        return m_bufferLength;\n" +
+            "    }\n\n" +
             "    std::uint64_t actingVersion() const SBE_NOEXCEPT\n" +
             "    {\n" +
             "        return m_actingVersion;\n" +