make node parsing and formatting ipv6-friendly

[retrnull]

There are two things that make using IPv6 nodes impossible:

• The formatting of the node address when passing it to monero, which does not include square brackets when the port is included
• The parsing of the node, which rejects IPv6 addresses because of the number of colons.

This patch fixes both issues by using the Guava library to parse and format addresses. Fixes #922