Another way in which the commit d768447 forced the client to wait for
the server to start was the UpdateDiagnosticNotifications call from the
FileReadyToParse which is called right after a buffer is loaded. In any
way if we don't have any previous FileReadyToParse request done for the
current file we bail out, so we we can wait for a FileReadyToParse
response to be available before asking if a completer is usable for the
current filetype.
ref: #1529
4104f7be82