Changelog

2.0.2

breaking: reactor: introduce a separate BackoffFactory interface for the first backoff

This only requires changes if you use retry policies in your own code, outside of the Policy.retry().

See #30. For some backoff policies, such as delegate and exponential policies, the first backoff was always 0, before next() was called. This is undesirable, and fixing it involved separating the backoff factory from the backoff itself.

The backoff classes, such as DelegateBackoff and ExponentialBackoff, now only have a next() method. The duration, which is now a property instead of a method, is only available after the first next() call.

For example, previously if you did this:
```
let backoff = new ExponentialBackoff();
while (!succeeded) {
  if (!tryAgain()) {
    await delay(backoff.duration());
    backoff = backoff.next();
  }
}
```
You now need to call next() before you access duration:
```
let backoff = new ExponentialBackoff();
while (!succeeded) {
  if (!tryAgain()) {
    backoff = backoff.next();
    await delay(backoff.duration);
  }
}
```
Note: if you use typescript, you will need another variable for it to understand you. Here’s an example of how we use it inside the RetryPolicy.

fix: events on the timeout policy being emitted incorrectly, or not emitted (see #27)

feat: add an optional CancellationToken to IPolicy.execute. Add cancellation awareness to all policies; see their specific documentation for more information. (see #25)
docs: fix outdated docs on Policy.circuitBreaker and unnecessary dashes in jsdoc comments (see #22, #23, #24)

breaking: Node versions <10 are no longer supported.
breaking: FallbackPolicy.onFallback is replaced with FallbackPolicy.onFailure. When a failure happens, a fallback will occur.
feat: add isBrokenCircuitError, isBulkheadRejectedError, isIsolatedCircuitError, isTaskCancelledError methods to the errors and matching predicate functions.
feat: all policies now include onFailure and onSuccess callbacks for monitoring purposes (see #20)
fix: add onHalfOpen event to the circuit breaker (see #18)
fix: retry.exponential() requiring an argument when it should have been optional (see #18)

feat: add .dangerouslyUnref methods for timeouts and retries (#11, thanks to @novemberborn)

fix: Timeout.Aggressive triggering timeouts immediately (#16, thanks to @ekillops)
fix: correctly compile to ES2018 (#10, thanks to @novemberborn)

Initial Release