Namecoin testnet beta

I would like to create a new branch on namecoin/libcoin. The branch would be namecoin-testnet-beta, the beta part is to indicate that it still uses some hacks to let it sync.
I think this should be done via a method in `Chain` (similar to `adhere_names` & friends), to follow libcoin's design spirit.  Something like `allow_incorrect_pow` (not sure yet what the thing we have to allow here really is, some name that describes it).