Added caching on _checkArchivers to improve performance

When adding lots of mount points (around 50 local folders) I experienced very poor performance. Every action took around 6 seconds.
The bottleneck is the _checkArchivers method which is called on every mount.
I added a simple session-based caching. There might be better methods for this, but it improved speed by factor 10 for me (around 0.5 secs instead of 6 secs).