Reference assembly loading fixes: 1) don't try to load assemblies already loaded, 2) Use ConcurrentDictionary in thread-safe way (#415)

Author: Thorium

src/ProvidedTypes.fs

@@ -9039,6 +9039,7 @@ namespace ProviderImplementation.ProvidedTypes
         let enqueueReferencedAssemblies(asm: Assembly) = 
             do sourceAssembliesQueue.Add (fun () -> 
                 [| for referencedAssemblyName  in asm.GetReferencedAssemblies() do
+                    if not (sourceAssembliesTable_.ContainsKey referencedAssemblyName.Name) then
                       let referencedAssembly = try Assembly.Load(referencedAssemblyName) with _ -> null
                       if not (isNull referencedAssembly) then
                           yield referencedAssembly |])
@@ -9052,12 +9053,12 @@ namespace ProviderImplementation.ProvidedTypes
                 for q in qs do 
                     for asm in q() do 
                         let simpleName = asm.GetName().Name
-                        if not (sourceAssembliesTable_.ContainsKey(simpleName)) then 
-                            sourceAssembliesTable_.[simpleName] <- asm
+                        sourceAssembliesTable_.GetOrAdd(simpleName, fun k ->
                             sourceAssemblies_.Add asm
                             // Find the transitive closure of all referenced assemblies
                             enqueueReferencedAssemblies asm
-
+                            asm
+                        ) |> ignore
             sourceAssemblies_
 
         /// When translating quotations, Expr.Var's are translated to new variable respecting reference equality.