`Workspace.documentServices` is never cleared up

[ahoppen]

Reading through code, I noticed that Workspace.documentServices only ever has elements added to it and we never remove elements from it, even when documents are closed.

What’s more is that setDocumentServices is called from places other than openDocument, so documentServices might contain more documents than the ones that have been opened by the client.

I think the correct thing to do here is to make SourceKitLSPServer.languageService just return an existing or newly started language service and only call setLanguageServices from openDocument. That way we don’t need to deal with race conditions for multiple calls to setLanguageServices anymore (openDocument is a blocker request for requests affecting that document URI) and we can remove documents from documentServices when a document is closed.