[C++] Add const to buffer accessor within a message or fixed flyweight. Issue aeron-io#601.

Author: mjpt777

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

@@ -1427,7 +1427,7 @@ private CharSequence generateFixedFlyweightCode(final String className, final in
  " {\n" +
  " return m_offset;\n" +
  " }\n\n" +
- " char *buffer() SBE_NOEXCEPT\n" +
+ " const char * buffer() const SBE_NOEXCEPT\n" +
  " {\n" +
  " return m_buffer;\n" +
  " }\n\n" +
@@ -1580,7 +1580,7 @@ private CharSequence generateMessageFlyweightCode(final String className, final
  " {\n" +
  " return sbePosition() - m_offset;\n" +
  " }\n\n" +
- " char *buffer() SBE_NOEXCEPT\n" +
+ " const char * buffer() SBE_NOEXCEPT\n" +
  " {\n" +
  " return m_buffer;\n" +
  " }\n\n" +
@@ -1697,7 +1697,7 @@ private static void generateFieldMetaAttributeMethod(
  final String semanticType = encoding.semanticType() == null ? "" : encoding.semanticType();
 
  sb.append(String.format("\n" +
- indent + " static const char *%sMetaAttribute(const ::sbe::MetaAttribute::Attribute metaAttribute)" +
+ indent + " static const char * %sMetaAttribute(const ::sbe::MetaAttribute::Attribute metaAttribute)" +
  " SBE_NOEXCEPT\n" +
  indent + " {\n" +
  indent + " switch (metaAttribute)\n" +