Add concurrent objects check

Here we use `IHasColumn` to support rulesets with columns, and so I moved that interface out into `osu.Game` from `osu.Game.Rulesets.Mania`.

We also use the same threshold as the unsnap check to ensure that no problems slip through. Specifically where an object is simultaneously not concurrent and not unsnapped but still on the same tick.

osu.Game/Rulesets/Objects/Types/IHasColumn.cs

@@ -0,0 +1,16 @@
+// Copyright (c) ppy Pty Ltd <contact@ppy.sh>. Licensed under the MIT Licence.
+// See the LICENCE file in the repository root for full licence text.
+
+namespace osu.Game.Rulesets.Objects.Types
+{
+    /// <summary>
+    /// A type of hit object which lies in one of a number of predetermined columns.
+    /// </summary>
+    public interface IHasColumn
+    {
+        /// <summary>
+        /// The column which the hit object lies in.
+        /// </summary>
+        int Column { get; }
+    }
+}