"Sprinting" mobs stop sprinting upon reloading the world

Various mobs which can "sprint" (move faster than their idle walking speed) will cease doing so when the world is reloaded. This affects:

• Villagers and animals fleeing from danger
• Creepers fleeing from cats
• Pigs and striders which have been "speed-boosted" by their riders
• Various situations covered by MC-250059, if we consider mobs like pillagers, iron golems, etc. to "walk" when idle and "sprint" when pursuing a target, which makes sense to me

This happens because mobs' sprinting state is not saved to NBT.

How to reproduce

1. Punch a cow
2. Immediately reload the world
   Notice how the cow is no longer fleeing from you. (It also forgot where it was walking to due to MC-183791.)

Suggested fix

I suggest fixing this as part of a refactor that also fixes MC-183791 and MC-250059. Extra speed from sprinting would be stored as a modifier to the minecraft:movement_speed attribute (as would player sprinting/crouching/crawling), and the base Mob class would gain 3 new fields: danger_source (int array; UUID) attack_target (int array; UUID) and path_target (int array; block pos). Whenever a mob re-evaluates its movement, it checks these fields in the order listed here to determine what to do. Pigs and striders would also gain boost_time_remaining and boost_time_total fields so that their boost state is preserved.

I realize that this section kind of reads like a design suggestion but I feel like I would be remiss not to propose a comprehensive solution. As far as I know Mojang aspires for gameplay to be completely unaffected by world reloads – with very limited exceptions – so these internal changes seem to me like the natural extension of that logic.