Add support for getaddednodeinfo RPC command.

This commit adds full support for the getaddednodeinfo RPC command
including DNS lookups which abide by proxy/onion/tor rules when the DNS
flag is specified.  Note that it returns an array of strings when the DNS
flag is not set which is different than the current version of bitcoind
which is bugged and scheduled to be fixed per issue 3581 on the bitcoind
issue tracker.
Add getaddednodeinfo support to btcctl.