refactor!(daemon): Rename mutex to serial-jobs

[rekmarks]

Renames the daemon's Mutex to SerialJobs, which better reflects its purpose. Also removes lock() and unlock() from its interface.

[kriskowal]

I’m not quite satisfied. This is something that might graduate to @endo/promise-kit someday, a pair of “builder objects” for parallel and serial aggregate jobs. Consider SerialJobs vs ParallelJobs.

Here’s an animation monad library for a game I wrote once that had analogous primitives: https://github.com/kriskowal/peruacru/blob/master/animation.js#L119-L130 The meaningful difference is that animations get scheduled when JavaScript yields to the browser’s outer render loop, so unlike promises, this is a strict monad (no auto-unwrap) and it’s synchronous. It is analogous to promises in many other ways.