Refactor TimeUtil: use Java 8 Date-Time API and improve time intervals

[bertm]

The time intervals were subtly broken:

• Parsing 0.1s would yield 1 ms instead of the correct 100 ms. Fix this by parsing the seconds.millis as fractional seconds.
• Formatting would format fractional seconds locale-sensitively where parsing would assume point as a decimal separator, so toMillis(fromMillis(...)) would fail depending on locale. Fix this by always formatting in the root locale (decimal point).

As a tiny bonus, time interval parsing is now approximately 3x faster.