Handle LSP requests when blocking while flushing writes

[the-mikedavis]

#11902 (comment) points out a case where a language server appears to be awaiting a response to workspace/configuration in order to respond to textDocument/formatting. This is a problem for :write-quit since we block the main thread waiting for the textDocument/formatting response while the main thread is also responsible for replying to LSP server requests. Ideally we should be able to continue responding to LSP requests while awaiting shutdown.

[pascalkuthe]

yeah I suspected that our write flushing would eventually be an issue somewhere. We should not be shutting down the editor until writes are done. Ideally we would even show something that indicates the editor is still saving (and allow cancelling the shutdown ala #6436)