Add a schedule-aware repeat

[rucek]

While the retry mechanism allows for repeating an operation - according to a Schedule in case of error, we'd also like to have a schedule-aware repeat, which would run an operation according to a Schedule or perhaps until a stop condition is met.

Some things to consider:

• what happens if the operation fails before the scheduled number of repetitions? Maybe this could be expressed with a multi-policy (see: Add multi-policies for retries #55) that combines repeat and retry?
• perhaps the stop condition could be thought of yet another Schedule like UntilResult[T](p: T => Boolean) or Until(p: => Boolean), or both?

[adamw]

Some operations that we'd like to have:

• schedule at fixed rate
• schedule with an initial delay

Should we measure time between start of each operation (start2-start1), or the pause between operation invocations (start2-end1)? Or maybe this should be configurable?