Units idling in GATHER APPROACHING and RETURN RESOURCE

[elexis]

In this danubius 3v3 replay with r22537, at minute 5 I build a storehouse in front of my CC, very close to the trees with many units around (so a pathfinding stresstest) and it resulted in units walking around for half a minute and then standing around doing nothing while being in GATHER APPROACHING and RETURN RESOURCE states until I found them and ordered them to do something else.

[wraitii]

I don't think this is a release blocker per-se, so downgrading to Must Have:

• This is sort of an extension of the case where units try to gather from an unreachable tree, which already happened before A23, and it's a pretty rare occurrence.
• It's also technically not a new issue since it could happen before A23, though it's probably more likely now.
• It's also self-fixing since the units would have carried on gathering once the offending tree was cut off.

[wraitii]

In 22816:

Improve gathering behaviour when a target is not reachable

Units can be stuck in gather states when the target ought to be collectable from, but it's actually unreachable.

A new FINDINGNEWTARGET intermediate state for GATHER will try and find a new, different target to collect from. This should generally improve the behaviour (perfect reachability checks would be required to completely fix it).
Use this also when the target is knowably unreachable / uncollectible.

Fixes #5529

Differential Revision: ​https://code.wildfiregames.com/D2120