Add new event and modify some workflow

[pedrommone]

Description

I've added a new event AttemptSucceded and changed the way events are fired.

Now we have two main events:

• When the service is restored/braked;
• When the service get an attempt (failed or not).

They are split into four events:

• AttemptFailed fired by attempt method;
• AttemptSucceeded fired by attempt method;
• ServiceFailed fired by attempt method when the previous state is changed;
• ServiceRestored fired by attempt method when the previous state is changed.

Motivation and context

We need to notify our slack uptime channel when circuit breaker is fired or a service is restored.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.
• Each individual commit in the pull request is meaningful.
• I have added tests to cover my changes.
• If my change requires a change to the documentation, I have updated it accordingly.

Since the package don't have tests for event firing, I haven't built them.

[pedrommone]

@francescomalatesta I think this MR should be squashed.