DNS: more consistent and realistic handling of DNS events

[klimeryk]

Changes inspired by a comment by @stephanethomas.
Previously, we optimistically deleted the DNS record. Now, we wait for the response from the API. Same for adding DNS records.
The DNS_*_COMPLETED events are now only fired when there's no error, making reducer implementation more straightforward.

I did not add the DNS_DELETE and DNS_ADD events, since they are not needed right now and might suggest that something is happening (or should be) when they are fired. If someone will need them (for example, when implementing optimistic versions of those actions), it's trivial to add them.

While getting rid of the onComplete callback and keeping everything pure was tempting (as discussed with Stephane), looking at the fluxing (ha-ha, get it?) approach to Redux usage right now in Calypso, I decided it's beyond my abilities at the moment. But I'll keep a close eye on the developments in this area and when I'll educate myself and have some spare cycles I'll get back to this :)

cc:

• @dzver (although, I'm sure he already felt the disturbance in the Force)
• @aidvu (sorry it's not a bigger change that you could nitpick easily, like I did 😁)

[dzver]

👍

I cannot help but notice how the recent changes of the DNS editor have significantly worsened the UX. It's not supposed to work like it works right now.

1. When you click add, the button needs to become gray/disabled
2. When you click delete, it needs to disappear immediately.

[klimeryk]

I agree that the DNS actions are not as responsive as they could be. I'll merge this as-is now, and will try to improve this situation in some spare time.