move database utils to Database/Utils, create DatabaseMigrator

PluralKit.Core/Database/Utils/DbConnectionCountHolder.cs

@@ -0,0 +1,20 @@
+using System.Threading;
+
+namespace PluralKit.Core
+{
+    public class DbConnectionCountHolder
+    {
+        private int _connectionCount;
+        public int ConnectionCount => _connectionCount;
+
+        public void Increment()
+        {
+            Interlocked.Increment(ref _connectionCount);
+        }
+
+        public void Decrement()
+        {
+            Interlocked.Decrement(ref _connectionCount);
+        }
+    }
+}