Set lower feecap on PoSt messages with low balance

[magik6k]

More testing definitely todo

[Kubuxu]

This should for sure be more than 2, if you are setting Premium for 5 block inclusion.

[magik6k]

Set it a bit higher, bust still pretty low - this is estimating the lowest fee we'll set on the message if we don't have enough funds in any account to set more. If we have enough funds, we'll use the default estimation from GasEstimateMessageGas above

[whyrusleeping]

I feel like this method could be written more cleanly as something like:

for _,a := range append(controlAddrs, owner, worker) {
  ok, err := maybeUseAddress(a, &leastBad, &bestAvail)
  if ok {
   return leastBad, bestAvail
  }
}

[arajasek]

This should work, but I'm confused about the logic. We seem to be doing:

- if we find any address with more than `goodFunds`, use that address
- if not, of all the addresses that have more than `minFunds`, use the one with most balance
- if no such addresses exist, eff it, use worker (even if there's a control address with greater balance than worker)

Why not do:

- if we find any address with more than `goodFunds`, use that address
- if not, use the address that has the most balance (regardless of whether it is better than `minFunds` or not)

The change would probably be to initialize bestAvail to worker's balance at line 39, and remove the worker from the list of addresses we call maybeUseAddress with (and remove the special-case in 53-61).