Prepare for using multiple blockchain sources for sweep

- Extracted private functions for fetch, parse and balance calc
- Put the dogechain.info unspent url in a static string
- Removed leftover code that made fetch take at least 1500ms
- Added state NOTHING_TO_DO to diff between errors and 0 vouts
- only process vouts if fetch/parse was successful
- some minor syntax improvements
Add sochain as a fallback blockchain source for sweep
Randomly use either sochain or dogechain for sweep

This spreads the load between the 2 providers. If another
blockchain provider pops up, adding it to the url list
should suffice.
Merge pull request #25 from patricklodder/sweep-multisource

Fetch unspent info from one of multiple source source options + fallback