flexlock-cb (npm): A locking library like [`mutexify`](https://github.com/mafintosh/mutexify), [`mutex-js`](https://github.com/danielglennross/mutex-js), [`await-lock`](https://www.npmjs.com/package/await-lock), and [many more](https://www.npmjs.com/search?q=promise+lock), 
Keywords: promise|lock|async|javascript|multithreading|mutex|mutex-lock|process