Drop copy constructors creating non-owning variants (stream_t, event_t)

[eyalroz]

Some of our wrappers, which come in both an owning and a non-owning variant, have a copy ctor creating a non-owning variants. While that may be convenient - on second thought, it is too unsafe. Users may make a copy of a local or temporary variable, keep the copy and let the original die - resulting in the equivalent of a dangling reference. In particular, this happens with std::vector's due to our destructors being throwing - something recently exposed via #426 .

So - it's bye-bye copy construction. See also #406 .