Tweak the code, comments and utilities.

[lrhn]

Nothing here should change behavior or API, only implementation and presentation.

Introduced the Mutex class because Pool seemed overqualified/overcomplicated for the job of being a single-lock mutex.

Added a JsonConvertible interface, just to have an extension method on Iterable<JsonConvertible> for doing [for (var v in iterable) v.toJson()]. Happened too often, so a good candidate for a helper function.

Every change here is a suggestion, none of it is essential. Feel free to ignore any parts that seem unnecessary. (The task-URI cache is probably unnecessary.)