Garbage collection

[tardieu]

MCAD caches limited information about AppWrappers in maps keyed by the AppWrapper UID. The cached information is cleared when we finalize the deletion of the AppWrapper by removing the finalizer. It is possible for the cached information to leak, i.e., remain in memory post deletion, if the finalizer is deleted outside of MCAD. MCAD needs a garbage collection mechanism to purge cached entries for non-existent UIDs.