Allows implementations to preempt further iteration of this iterator in the current RPC.
Implementations can use the yield method on the callback to instruct the caller to cease
collecting more results within this RPC. An implementation would only need to implement this
mechanism if a next or seek call has been taking so long as to starve out other scans within
the same thread pool. Most iterators do not need to implement this method. The yield method on
the callback accepts a Key which will be used as the start key (non-inclusive) on the seek call
in the next RPC. This feature is not supported for isolated scans.