* Core/Units: add new functionality for units to block or enable combat entirely via helper to reflect what CREATURE_DIFFICULTYFLAGS_IGNORE_COMBAT and client AI functions imply
* yeah...
* Nuke Creature::IsCombatDisallowed helper as its unused by now
* no combat extra flag may now be changed on transforms if the transformed entry does not have the flag
* Core/Combat: Disable triggers entering combat
* Move check to database flags_extra flag in creature_template with value 0x00002000
* Fix Rotface puddle stalker too
* Rename 9999_99_99_99_world.sql to 2020_07_22_00_world.sql
- PvE combat is now always mutual. UNIT_FLAG_IN_COMBAT is backed by actual references to the units we're in combat with.
- PvP combat is now also tracked, and almost always mutual; spells like Vanish and Feign Death can break this rule. That means we can easily determine a list of players we're fighting.
- By extension, IsInCombatWith now has sensible behavior when invoked on nonplayers.
- Threat and combat systems are no longer the same.
- They still have an enforced relationship (threat implies combat - clearing combat clears threat)...
- ...but we can have combat without threat. A creature (with threat list) isn't considered to be engaged until it has an entry on its threat list...
- ...which means we can now faithfully replicate retail engage behavior. Combat on projectile launch - engagement start on projectile impact. Yay for progress!
- AI method refactor, as already ported in 6113b9d - `JustEngagedWith`, `JustEnteredCombat` and `JustExitedCombat`.
- Vehicle threat is now properly pooled on the main vehicle body (fixes#16542).
- Various edge case bug fixes for threat redirects (Misdirection "cancelling" Vigilance and similar).
- Target re-selection is now significantly faster.
- Fixed a ton of other smaller edge case bugs, probably.
Closes#7951 and #19998.
Shauren