- Catch the CancellationException so it's not propagated
- Improve performance by using a cache so we don't recompute state for
unmodified branches of the resource tree.
- Extracted a method to remove nodes from cache, but keep cache
consistent.

Signed-off-by: Mickael Istria <mistria@redhat.com>
Change-Id: I40bb19d58447f2789e80ecb515fdbf9edd6be63f