* Fix checkParentHeadersExist and cover pruning_violation_proof_of_work_and_difficulty.go with tests
* Remove unused variable
* Change consensus violation
* Change condition order
* Get rid of irrelevant error codes in extractRejectCode
* Fix wrong test db names
* Fix checkParentHeadersExist
* Log the hashrate of each block
* Add a test for GetHashrateString
* Move difficulty related functions to its own package
* Convert the validated log in validateAndInsertBlock to a log function
* Add tests for max/min int