http_request: use class FailurePtr
http_request: use class FailurePtr

This eliminates the need for postponing the lease release (since
commit 05d3b942126df900a8761babd83f53bc422ba695), because class
FailurePtr lets us manipulate the FailureInfo without having to
remember the server's SocketAddress.

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