feat: Adds a single threaded array pool (#967)

## Added Feature
Adds a single threaded array pool that works exactly the same as `ArrayPool<T>.Shared`.
The `STArrayPool<T>.Shared` can only be used on a single thread, the main game thread of the server.

Note: Unlike the built-in array pool, there is no hook into the _GC Gen 2_. This means to relieve potentially high memory pressure, `ArrayPool<T>.Shared.Trim()` must be called. The pool will only release arrays _after two successive calls within 10 seconds or longer_. If the server is at less than 70% total memory usage, or the server is not going to use this pool for something egregious, then don't bother ever calling Trim().


## Changes
- [X] Fixes ArrayPool calls that should be cleared due to references.
- [X] Benchmarks against ArrayPool with 4+ rented arrays deep of the same length.
- [x] Unit tests

Projects/Server/Maps/Map.cs

@@ -691,7 +691,7 @@ namespace Server
                 }
             }
 
-            ArrayPool<Item>.Shared.Return(items);
+            ArrayPool<Item>.Shared.Return(items, true);
         }
 
         /* This could probably be re-implemented if necessary (perhaps via an ITile interface?).