Refactor for Vim8/Neovim job support. - Refactor all modules' `generate` methods to use a vaguely generic job API wrapper that works for both Vim8 and Neovim jobs. - Make the `statusline` method use new `User` autocommands driven by the job-started/ended callbacks. - Remove all the lock-file-related stuff. - Better error/warning messages. - Move a few things around.