Updated DataType factory to use PluginTypeResolver and made the dictionary thread safe.

Updated MacroEngineFactory to use PluginTypeResolver and made it thread safe. Have also
updated it to only create one instance of each IMacroEngine per application which should
increase performance a teeny bit, plus that's how it should be.

src/Umbraco.Core/Resolving/MultipleResolverBase.cs

@@ -3,6 +3,9 @@ using System.Threading;
 
 namespace Umbraco.Core.Resolving
 {
+	//NOTE: This class should also support creating instances of the object and managing their lifespan,
+	// for example, some resolvers would want to return new object each time, per request or per application lifetime.
+
 	/// <summary>
 	/// A Resolver to return and set a Multiply registered object.
 	/// </summary>