Add support for dynamic monster hate durations.

[tristandunn]

The ability for monsters to quickly forget attackers or remember them long-term would allow them to be more believable.

• The duration should be stored on monsters and spawns.
  • Instead of adding a duration-specific field, a metadata field should be considered on both. It can be used for any other future information that doesn't necessarily need to be queryable.
• Limits should be considered when setting them, but no validation is required. Worse case scenario it'd expire earlier than expected.