Use iteratorWithSnapshot in the select methods

Since UIMA 2.7.0, there are "iterateWithSnapshot" methods that safeguard against modifying the indices while iterating over them. uimaFIT could avoid concurrency problems in client code by consistently using these new methods at the expense of some performances. I would expect though, that whoever wants to the the last piece of performance out of the code would not use the uimaFIT select methods anyway - they would try to optimize iteration order at all cost.