* remove difficulty field from block_header (derive from total_difficulty)
* address feedback on PR and general cleanup
* rustfmt
* rework pow/difficulty validation in pipe::validate_header
now that we only have total_difficulty available
* cleanup various todos
* rustfmt
* rework DifficultyIterator to track header and prev_header state
* rustfmt caught some garbage syntax
* cleanup
* Adding ability to load multiple mining plugins in parallel via cuckoo miner
* updating with newest cuckoo-miner changes
* revert default config value
* update pow document with link to podcast
* Clarifications to SHA3 hash and addition of progress-freeness concept in Pooling Capacity
* Update to cuckoo cycle details based on John Tromp's comments.
* A couple of typos
* More clarifications based on John Tromp's feedback
* Beginning to add a POW description, and some minor changes to mining testing, addition of further debug information to mining output.
* Many additions to create first draft of POW documentation
* Fixes to difficult adjustments by adding a MINIMUM_DIFFICULTY consensus value. Otherwise never adjusted above 1 due to flooring.
