Batched Packed and Distributed Allocations (#804)

* Batched Packed and Distributed Allocations

This implements a batching algorithm for Packed and Distributed
GameServerAllocations (GSA).

This ensures that we can still get high throughout on GSA's, while
retaining a tight packing of Allocated GameServers on each node with
the Packed strategy.

The Distributed strategy is now implemented with a randomised Allocation
process, so ensure distributed load under this strategy.

Closes #783

* Move updateQueue into allocationUpdateWorkers.

* Add large explanatory code block.

pkg/gameserverallocations/cache.go

@@ -70,3 +70,10 @@ func (e *gameServerCacheEntry) Range(f func(key string, gs *stablev1alpha1.GameS
 		}
 	}
 }
+
+// Len returns the current length of the cache
+func (e *gameServerCacheEntry) Len() int {
+	e.mu.RLock()
+	defer e.mu.RUnlock()
+	return len(e.cache)
+}