* Adding cuckoo-miner into grin as a proper dependency now
* Defaulting to using cuckoo-miner
* Updates to tests to use cuckoo_miner by default, (using cuckoo16)
Introduce a new Chain struct that maintains the current head,
acts as facade to the store and the block processing pipeline
and handles all appropriate locking.
All higher level components, like the overall server, REST APIs
and miner have been update to only depend on Chain. Cleaned up
some duplication and tests now that there's a single entry point
to all blockchain operations.
Integrate the new difficulty calculation into the block chain
validation, the miner and tests. As the difficulty calculation
doesn't use varying Cuckoo sizes anymore and we narrowed down
reasonable final Cuckoo Cycle parameters, removed all Cuckoo
Cycle sizes from block headers.
Formalized easier Cuckoo Cycle sizes for testing (and possibly
testnet) by introducing a test mode in configuration. Updated
all tests.
