XMPP room connection handler

[ForNeVeR]

Consider this scenario:

1. Client connects to XMPP, but not yet entered the room
2. Client receives a message from Telegram
3. Client tries to send a message to the room and fails
4. Client connects to the XMPP room

In that case, the message will be lost. We need to prevent cases like this. Probably with small async magic when connecting to the room.

[ForNeVeR]

Blocked on #29.

[ForNeVeR]

It is a cause of real incidents such as #54. I'm raising the priority.

[ForNeVeR]

Alrighty, I'll dump some current thoughts here. I want to create some sort of async wrapper around the XMPP client, so we'll be able to call something like that:

let client = AsyncClient.create()
async {
  do! client.Connect server
  do! client.SignIn credentials
  do! client.EnterRoom roomSettings
  do! client.Run()
}

Where Run() should call some sort of event handlers where each handler itself may be async, too.

There're problems with error handling though.

Some of the methods may throw exceptions (e.g. SignIn will throw if it cannot sign in, EnterRoom will throw if it cannot enter the room), that's simple and we know how to handle that. But some methods establish a context where an error may be thrown at any moment.

For example, after I execute client.Connect(), the server may disconnect me literally at any moment, thus terminating the whole workflow (and the disconnect should terminate the whole async workflow actually). And client.EnterRoom() establishes the room context which should be terminated after the bot leaves the room (e.g. by being kicked).

This concept of contexts that may be terminated at any moment with an error is complex, and I don't know how to properly encode that. Probably may be encoded using child asyncronous workflows in F#? I don't know because I didn't worked with them much.

In my earlier notes, I was thinking about something like that:

client.SignIn(sessionLifetime => {
  client.EnterRoom(sessionLifetime, roomLifetime => {
    client.SendRoomMessage(roomLifetime, "text");
  });
});

Where each stage has its own "lifetime", the lifetimes are nested and terminated properly. But I'm not sure about that now.

Also, there may be problems with our current approach because of the race conditions when accessing the client output stream. I'll need to think about that, too.

[ForNeVeR]

@ForNeVeR please investigate thread safety of our current solution and raise another issue about that. Probably we'll need to solve that first, but this is an issue about XMPP connection/room session API.