G1: Clear CLD claim marks concurrently

Currently G1 clears the CLD claim marks during the concurrent start pause.

This can be avoided by having the concurrent mark clear the CLD claim marks at the end of the concurrent phase (concurrently), restoring the original claimed marks state before the next concurrent start pause.

Full GC and concurrent undo need to do this as well (another time), but this work is either supposed to be negligible (full gc) or concurrent (concurrent undo) too.