Improve roslyn MetadataReference performance

- Add additional Caching in CachingMetadataReference to avoid poking the filesystem for last write time when the metadata of a MetadataReference, which is done a lot of times when looking up for symbols via the SymbolFinder class.
- Added parallelization of a MetadataReference caching

src/Uno.SourceGeneration.Engine.Shared/SourceGeneratorEngine.cs

@@ -59,20 +59,24 @@ namespace Uno.SourceGeneration.Host
 
 		private void PreInit()
 		{
-			// Used by MSB.ProjectCollection
-			ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
-
-			// Parallelize the initialization of the MefServices graph (Used by AdHocWorkspace)
-			Microsoft.CodeAnalysis.Host.Mef.MefHostServices.DefaultHost.ToString();
-
-			// Pre-load known references
-			if (_environment.ReferencePath != null)
+			Task.Run(() =>
 			{
-				foreach (var reference in _environment.ReferencePath)
+				// Used by MSB.ProjectCollection
+				ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
+
+				// Parallelize the initialization of the MefServices graph (Used by AdHocWorkspace)
+				Microsoft.CodeAnalysis.Host.Mef.MefHostServices.DefaultHost.ToString();
+
+				// Pre-load known references in parallel
+				if (_environment.ReferencePath != null)
 				{
-					_metadataResolver.ResolveReference(Path.GetFileName(reference), Path.GetDirectoryName(reference), new MetadataReferenceProperties());
+					_environment
+						.ReferencePath
+						.AsParallel()
+						.Select(r => _metadataResolver.ResolveReference(Path.GetFileName(r), Path.GetDirectoryName(r), new MetadataReferenceProperties()))
+						.ToArray();
 				}
-			}
+			});
 		}
 
 		public string[] Generate()

src/Uno.SourceGeneration.Host/Server/MetadataCache.cs

@@ -110,8 +110,9 @@ namespace Uno.SourceGeneration.Host.Server
     internal sealed class CachingMetadataReference : PortableExecutableReference
     {
         private static readonly MetadataAndSymbolCache s_mdCache = new MetadataAndSymbolCache();
+		private Metadata _metadata;
 
-        public CachingMetadataReference(string fullPath, MetadataReferenceProperties properties)
+		public CachingMetadataReference(string fullPath, MetadataReferenceProperties properties)
             : base(properties, fullPath)
         {
         }
@@ -123,7 +124,16 @@ namespace Uno.SourceGeneration.Host.Server
 
         protected override Metadata GetMetadataImpl()
         {
-            return s_mdCache.GetMetadata(FilePath, Properties);
+			if (_metadata == null)
+			{
+				// Locally cache the value for this instance, when a resolver will get the file a subsequent time
+				// the cache will validate for a newer version of the metadata. This avoids re-reading the target
+				// file time while a given compilation is running.
+				// See https://github.com/dotnet/roslyn/issues/34072
+				_metadata = s_mdCache.GetMetadata(FilePath, Properties);
+			}
+
+			return _metadata;
         }
 
         protected override PortableExecutableReference WithPropertiesImpl(MetadataReferenceProperties properties)