Good default value for buyback interval

[mdyring]

Let's discuss what a good default value of the buyback interval should be:

genesis.json:

    "buyback": {
      "interval": "1ms"
    },

I couldn't (with a quick search) figure out if this is actually used. But assuming it is, this seems to indicate buyback will be attempted as each new block is created.

Perhaps this is overly aggressive if there is computational overhead involved and we start to see a larger number of transactions triggering block creation in rapid succession. Maybe we should align it with the inflation interval? (60 seconds AFAIR)

@haasted @blewater