Custom parameters for generic operations about gneiss

[jklmnn]

I noticed that this approach is problematic for the block interface. It defines the Request type in Gneiss.Block.Client which depends on the Context type. Since the requests are often part of the application state the Context type also depends on the Request type creating a circular dependency. A solution could be changing the block interface accordingly, that all types containing relevant state should be defined in the session package and not inside its children. I would omit the block session for now since we do not have it on Linux and therefore cannot prove it.

[jklmnn]

Server.Finalize and Server.Initialize are only called by Dispatcher.Session_Cleanup and Dispatcher.Session_Initialize. They also need a context argument.

On Linux currently Finalize and the server event are both called by the same callback. And both get the Server_Session as argument. If Finalize is to be called by Session_Cleanup which only can be called from within Dispatch, the event needs to pass both the server and the dispatcher session. To avoid aliasing there should be two events, one for an IO event and one for an error event. The first will call the server event and the latter will call dispatch. Both get their own session parameter. Also both will get the file descriptor that called them passed.

[jklmnn]

Fixed by #173.