Add interface for database context factory

osu.Game/Database/IDatabaseContextFactory.cs

@@ -0,0 +1,20 @@
+// Copyright (c) 2007-2018 ppy Pty Ltd <contact@ppy.sh>.
+// Licensed under the MIT Licence - https://raw.githubusercontent.com/ppy/osu/master/LICENCE
+
+namespace osu.Game.Database
+{
+    public interface IDatabaseContextFactory
+    {
+        /// <summary>
+        /// Get a context for read-only usage.
+        /// </summary>
+        OsuDbContext Get();
+
+        /// <summary>
+        /// Request a context for write usage. Can be consumed in a nested fashion (and will return the same underlying context).
+        /// This method may block if a write is already active on a different thread.
+        /// </summary>
+        /// <returns>A usage containing a usable context.</returns>
+        DatabaseWriteUsage GetForWrite();
+    }
+}