Use TimeUnit constants instead of calculating milliseconds.

This replaces many occurences of “60 * 1000” (and all kinds of other durations) by “MINUTES.toMillis(1)” (and the matching other constants). In most places this changes an implicit `int` (60 * 1000 is an `int`) to `long`s (because that is what `TimeUnit.toMillis(…)` returns). Often this change is uncritical because the value is only used in private member variables, or on-the-fly in comparisons. However, a couple of interface methods had to be changed to accept a `long` as a value for e.g. a timeout (`BaseSender` immediately comes to mind). I also have no idea whether I accidentally touched one of the classes that are stored in db4o.

The changes have only been tested by the compiler and the unit tests, I did not yet try to run this.