Write fee estimate and peers files only when initialized

Fixes #4669 (see discussion there).

Also move the loading of addresses to StartNode() to make it more self-contained.
Alright, but at least the variable name is missleading? At this stage we could have a missing file, an up-version file or some serialisation error, which would make me think ``fFeeEstimatesInitialized`` shouldn't be true. Perhaps ``fWriteFeeEstimates`` is better.