Intern command strings

Only one method interns string because it is used during domain work,
parameterless version is used during domain upgrade and such commands
will only bloat string cache with single-use strings

Author: alex-kulakov

Orm/Xtensive.Orm/Sql/Compiler/SqlCompilationResult.cs

@@ -59,8 +59,8 @@ public string GetCommandText()
     public string GetCommandText(SqlPostCompilerConfiguration configuration)
     {
       if (resultText!=null)
-        return resultText;
-      string result = PostCompiler.Process(resultNodes, configuration, lastResultLength);
+        return string.Intern(resultText);
+      string result = string.Intern(PostCompiler.Process(resultNodes, configuration, lastResultLength));
       lastResultLength = result.Length;
       return result;
     }