s3:registry avoid updating keys which are going to be deleted in

s3:registry avoid updating keys which are going to be deleted in
reg_deletekey_recursive

this changes the complexity from O(n^2) to O(n) and reduces the time of
a 'net conf drop' with 10000 shares from 6min to 1.5s

Signed-off-by: Michael Adam <obnox@samba.org>