lib: rename kMaxCallbacksUntilQueueIsShortened

PR-URL: https://github.com/nodejs/node/pull/11473
Ref: https://github.com/nodejs/node/pull/11199#issuecomment-281184439


Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>