A `OneShotHandler` without any ongoing requests should not keep the
underlying connection alive indefinitely.

The `OneShotHandler` `keep_alive` property is altered on incoming and
outgoing reqeusts. By default it is initialized in `KeepAlive::Yes`. In
case there are no incoming or outgoing requests happening, this state is
never changed and thus the handler keeps the underlying connection alive
indefinitely.

With this commit the handler is initialized with `KeepAlive::Until`. As
before the `keep_alive` timer is updated on incoming requests and set to
`KeepAlive::Yes` on outgoing requests.

A `ProtocolsHandler` can be created before the underlying connection is
established. Thus setting a keep alive timeout might be problematic.
Instead set `keep_alive` to `Yes` at construction and alter it within
`ProtocolsHandler::poll`.