zone events: split the source file

The src/knot/zone/events.c file has grown terribly.

Let's split it into:

• zone events scheduling
• zone events implementation
• zone events replanning

Don't do this before release, I have two branches based on master.

Two branches dealing with the zone events?

I'm not sure, but @mvavrusa probably started some splitting already.

I'll deal with it, but if you could wait I wouldn't protest.

Reassigned to @jkadlec

Nah, I've just made some changes (117198ff) to accommodate for update forwarding.

Status changed to closed by commit b91322

Milestone changed to 1.6