Reset relay bandwidths when their IPv6 address changes

We implemented this for IPv4, but not IPv6.

From https://github.com/torproject/sbws/issues/202:

on Jun 18, 2018

Split off #154.

When IPv6 is more than a few percent of relay load, sbws should reset relay bandwidths on IPv6 address changes as well. (Tor doesn't measure IPv6 entry bandwidth separately, but we want to.)

Here's what we'd need to do:

    Add a relay or_addresses attribute
    Add or_addresses to result
    Check any IPv6 addresses in or_addresses to see if they've changed

We should also store IPv6, since currently sbws is only storing IPv4 addresses.