MultiPutRequest.toString is too expensive

[tsuna]

MultiPutRequest.toString is too expensive because it serializes every single edit in the MultiPutRequest. This method is called by the constructor of NotServingRegionException. In a high-throughput environment, this can waste a very significant amount of CPU cycles and cause a lot of unnecessary stress on the GC. We need to find a way to make MultiPutRequest.toString cheaper but still informative. I'm thinking of something based on the number and size of edits in the MultiPutRequest. Maybe just the size would be sufficient. If the MultiPutRequest contains less than 1k of data, render it as today, otherwise only show the first few items and then print how many items haven't been rendered in the string representation.