Memories might be shared among empires

**Describe the bug**
I think if multiple empires have memories of an object, they might all share the same memory because the `CopyObject` method in CopyingExtensions.cs is caching copies of objects and reusing them.

**Steps to reproduce**
1. Get three empires in the same star system.

**Expected behavior**
Each empire should have its own unique memories of the other two empires' space objects.

**Actual behavior**
Each two empires will share a memory of the third empire's space objects.

**Screenshots**
N/A

**Related issues**
N/A

**Testing plan**
???

**Additional context**
We still might want caching to avoid too many duplicate objects being created, but it should be keyed by empire at least, and have a key for the game host too.