Implement name_scan

This implements name_scan in libcoin.  As discussed on the forum (https://forum.namecoin.info/viewtopic.php?f=2&t=1917), it does not return the names in the same order as namecoind.  But this does not change the ability to enumerate all names with it.

Please test if you can, but do not merge here - I will try to get Michael to merge upstream to libcoin/libcoin.