In the random sync algorithm used by the DNS node iterator, we first
pick a random tree and then perform one sync action on that tree. This
happens in a loop until any node is found. If no trees contain any
nodes, the iterator would just keep starting over in a hot loop,
spinning at 100% CPU.

The fix is a bit complicated. The iterator now checks if any sync action
can be performed on the tree before selecting it. If no action can be
performed on any tree, it waits for the closest root record recheck time
to arrive and then tries again.