Implement Deadline for strategies

[pzmarzly]

The issue with $STRATEGY.take(n) is that is doesn't take function execution time into consideration. If you use a strategy that retries 100 times, every 10 milliseconds, but you use it against a function that takes 1 second to execute, the total execution time will be 101 seconds (100 * (fn execution + retry interval) = 100 * (1 + 0.01)).

In this PR I'm adding a deadline(max_duration) function, which restricts how much time can a strategy take. It uses Instant clock to count the time since the strategy was created, so it includes the function execution time.

Please tell me what you think.

[naomijub]

In case you want to add your contribution or have other suggestions: https://github.com/naomijub/tokio-retry