supporting prefer_not promotion rule

[shlomi-noach]

Up till now these promotion rules were supported:

• prefer: promote if possible
• neutral
• must_not: never even participates in promotion process.

We now add prefer_not:

neutral > prefer_not > must_not

A prefer_not server has nothing special about it; it is valid. It contains good data, but we wish not to promote it.

As an example, perhaps this is in particular a weak box, or some AWS box that is heavily used for something else.

However, if a master dies, and such box turns to be most up to date, then by all means we can use it in the promotion flow. For example, we can discretely promote it, let other boxes catch up, then, if promote a prefer box (aka candidate) or even a neutral box on top of it.

So basically it's similar to neutral in essence: nothing special about it, except we'd rather have neutral promoted instead of a prefer_not.