fix: avoid throwing exception in destructor

[metahys]

When data compression is enabled, CompressedOutput will be created in Client::Impl::SendData. If the socket is unfortunately broken during the execution of SocketOutput::DoWrite, a std::system_error exception will be thrown. Next, the destructor of CompressedOutput will be called during stack unwinding, and then Flush() in ~CompressedOutput will call SocketOutput::DoWrite again, eventually causing the program to terminate.

[CLAassistant]

All committers have signed the CLA.

[Enmk]

IMO it is better to remove Flush() from destructor altogether and make it a caller's responsibility to explicitly call Flush() as a happy-path. In this case all errors that arise are visible to the library user and no need to hide those.