OpenAlias support

[sorpaas]

This adds openalias (https://openalias.org/) support, hidden behind the openalias-support feature gate. This allows associating an address (be it BTC, XMR or ETC) to a domain name, so it is less likely to be hijacked and easier for users to read.

When it's enabled, on signing a transaction, it checks if the to address starts with 0x, if not, it will try to parse it as an openalias using the DNS TXT record.

An example of ETC's OpenAlias address can be found at donate.that.world:

$ dig -t txt donate.that.world

;; ANSWER SECTION:
donate.that.world.	97	IN	TXT	"oa1:etc recipient_address=0x001bAec5aDBF44d9d5A50B50D72e92921FCD528D; recipient_name=That World Donations;"