Scripts/Commands: Allow to set a custom duration to the Freeze command applied Aura

- Allow to set a default duration for the Freeze Aura in worldserver.conf
- Handle the .freeze and .unfreeze command custom effects through the Freeze AuraScript
- Make players able to eventually unfreeze by themselves since the aura will expire without someone having to use .unfreeze
- Make .freeze command usable with the following syntax:

      * .freeze (with a targeted player)
      * .freeze duration_in_seconds (with a targeted player)
      * .freeze playername duration (if duration is an invalid value it will be defaulted to the config one)
      * .freeze playername (in this case, duration will be defaulted to the config value)

- Make .listfreeze command able to show the remaining freeze duration on all affected players

Closes #12972
Scripts/Onyxia Lair: Some small improvements to GUID storage as it's only used locally
Scripts/Onyxia Lair: Slight change so we don't call Reset() in the 'ctor