[AVRO-3942] Mark MemoryOutputStream as final

[mkmkme]

AVRO-3942

What is the purpose of the change

Otherwise the class raises a compilation warning:

lang/c++/impl/Stream.cc:129:27: warning: class with destructor marked 'final' cannot be inherited from [-Wfinal-dtor-non-final-class]
  120 | class MemoryOutputStream : public OutputStream {
      |                          final
  121 | public:
  122 |     const size_t chunkSize_;
  123 |     std::vector<uint8_t *> data_;
  124 |     size_t available_;
  125 |     size_t byteCount_;
  126 |
  127 |     explicit MemoryOutputStream(size_t chunkSize) : chunkSize_(chunkSize),
  128 |                                                     available_(0), byteCount_(0) {}
  129 |     ~MemoryOutputStream() final {
      |                           ^
lang/c++/impl/Stream.cc:120:7: note: mark 'avro::MemoryOutputStream' as 'final' to silence this warning
  120 | class MemoryOutputStream : public OutputStream {
      |       ^

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Documentation

• Does this pull request introduce a new feature? no

[mkmkme]

This PR is effectively the same as #2550 which got closed because the fork was deleted.

[KalleOlaviNiemitalo]

OK, this file already uses final in function members, and both were introduced in C++11.

This change will cause std::is_final<MemoryOutputStream>::value to become true but I expect that will only be a good thing.

[martin-g]

Thank you, @mkmkme !