Loop with dynamic interval

[jpuh]

In some cases it might be desirable to control when the next loop runs, for instance:

• sooner because there's work to do, like pending tasks that could be processed;
• later because in some periods there's no need to run often.

To address these cases, this PR takes into account the loop callback return value:

• if the callback returns an object with interval key then it is used for the next loop;
• otherwise the default loop interval is used.

This is non breaking change.