Improvements in CChainParams

Replace direct references to RegTest() with constant parameters on CChainParams interface.
EDIT: dependency merged
I agree that this can probably be simplified and maybe some of the parameters I have created can be eliminated some time in the future, but I didn't want to alter the behaviour in this PR, just refactor. Now that the different special cases don't depend on testnet or regtest, it should be easier to think about them and simplify the code around the usage of the parameters.