[C++]: add non-const buffer method. Add const to const char * buffer methdo for message flyweight. For aeron-io#607.

Author: tmontgomery

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

@@ -1431,6 +1431,10 @@ private CharSequence generateFixedFlyweightCode(final String className, final in
  " {\n" +
  " return m_buffer;\n" +
  " }\n\n" +
+ " char * buffer() SBE_NOEXCEPT\n" +
+ " {\n" +
+ " return m_buffer;\n" +
+ " }\n\n" +
  " std::uint64_t bufferLength() const SBE_NOEXCEPT\n" +
  " {\n" +
  " return m_bufferLength;\n" +
@@ -1580,7 +1584,11 @@ private CharSequence generateMessageFlyweightCode(final String className, final
  " {\n" +
  " return sbePosition() - m_offset;\n" +
  " }\n\n" +
- " const char * buffer() SBE_NOEXCEPT\n" +
+ " const char * buffer() const SBE_NOEXCEPT\n" +
+ " {\n" +
+ " return m_buffer;\n" +
+ " }\n\n" +
+ " char * buffer() SBE_NOEXCEPT\n" +
  " {\n" +
  " return m_buffer;\n" +
  " }\n\n" +