Improve peer host name resolution (closes #360)

- Take a QString in argument to avoid converting it twice. The caller already
  has the IP as a QString.
- Fix bug where hostnames coming from the cache would not the displayed.
- Fix bug where the resolved hostnames would not be displayed if the IP we know
  does not match the first one in the QHostInfo::addresses() list. It may come
  later in the list or may be formatted differently (possible with IPv6).
- Avoid requesting again host names for IPs that were already requested recently
  but whose resolution did not return a useful hostname.