perf: Avoid expensive Plot#getOwner calls in Plot#getOwners

[Vrganj]

Overview

Description

Refactored Plot#getOwners and reduced the number of Plot#getOwner calls because
they go through a half dozen flag containers to check whether the plot is server-owned.

I'd like input on the first null check. The behavior is the same as the old implementation,
but it seems a little illogical. If there are 2 plots, one without an owner - getOwners would
return an empty set if called on the ownerless one, and 1 element on the other, creating
inconsistency.

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[SirYwell]

I assume the null check for the connected plots is rather an assert. The owner is only null if the plot is not claimed; and if it isn't claimed but merged with other plots at the same time, something is wrong already.

[Vrganj]

Ah, then I suppose one of null checks can be removed.