Adjust cache cleanup to the deficits of RRDP. Unlike rysnc, RRDP has no

Adjust cache cleanup to the deficits of RRDP. Unlike rysnc, RRDP has no
method to sync the cache. It just depends on all deltas to work but has
no method to check if the result is consistent with the source.
Because of this do not unlink files which were fetched via RRDP. Instead
move them back to their .rrdp directory and hope the will be cleaned up.
This should help to keep the cache coherent in some edge cases.
OK tb@