Default empty destructors

[ecatmur]

downstream: https://github.com/boostorg/asio/pull/407

The compiler-generated copy constructor and copy assignment operator are deprecated since C++11 on classes with user-declared destructors.

This change allows clean compilation with the -Wdeprecated-copy-dtor/-Wdeprecated-copy-with-user-provided-dtor flag.

cf. https://github.com/chriskohlhoff/asio/issues/792#issuecomment-1565179044