It is possible that at some point in the future users will want to write multi-threaded iterators. This could possibly introduce thread safety problems in the system iterators, especially in the case where multiple threads might be calling "next", "getTopKey", etc. at the same time. In order to maintain some semblance of sanity, we need to synchronize these methods at the top of the system iterator tree. Eventually, we might also want to change the iterator API to make it useful to call multiple of these methods concurrently, but that will be in another ticket.
The immediate solution calls for a simple iterator that is similar to the WrappingIterator called the SynchronizedIterator, all of whose public methods are synchronized on the object's monitor. In single-node performance testing we found the read performance cost of this solution to be below the noise threshold, approximately less than a 1% cost.