[MRESOLVER-278] Session close and onClose hooks fix (#207)

There are valid cases in Maven when interaction with
resolver (that would "lazily" init SyncContext) happens
way late, when session is already done readOnly.

Fix: adding onCloseHandler should NOT be affected by
readOnly state of session, as it merely means it's members
like LRM, etc are "fixed", but session data etc are all
still mutable.

This change makes all Maven ITs pass with this resolver.

Author: cstamas

maven-resolver-api/src/main/java/org/eclipse/aether/DefaultRepositorySystemSession.java

@@ -816,9 +816,9 @@ private void verifyStateForMutation()
         {
             throw new IllegalStateException( "repository system session is read-only" );
         }
-        if ( isClosed() )
+        if ( closed.get() )
         {
-            throw new IllegalStateException( "repository system session is closed" );
+            throw new IllegalStateException( "repository system session is already closed" );
         }
     }
 
@@ -894,8 +894,11 @@ public Collection<FileTransformer> getTransformersForArtifact( Artifact artifact
     @Override
     public void addOnCloseHandler( Consumer<RepositorySystemSession> handler )
     {
-        verifyStateForMutation();
         requireNonNull( handler, "handler cannot be null" );
+        if ( closed.get() )
+        {
+            throw new IllegalStateException( "repository system session is already closed" );
+        }
         onCloseHandlers.add( 0, handler );
     }