prop224: Refactor parse_extended_hostname() to parse v3 addrs.

We need this func so that we recognize SOCKS conns to v3 addresses.

- Also rename rend_valid_service_id() to rend_valid_v2_service_id()

- Also move parse_extended_hostname() tests to their own unittest, and
  add a v3 address to the test as well.

Signed-off-by: David Goulet <dgoulet@torproject.org>