http-cache: added async_operation implementation
http-cache: added async_operation implementation

The http-cache library will need more deinitialization in case abort()
is called.  Let's implement our own async_operation class.


CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy,CM4all/beng-proxy