Make sure polish() is reentrant.

Take a copy of the items currently scheduled for polish (and clear the internal
list) to ensure an item can re-add itself to the polish list during an
updatePolish().

Change-Id: I0c629795f64d5926ac87a838ea5e216b6a1fa6cf
Reviewed-by: Gunnar Sletta <gunnar.sletta@nokia.com>