RFR-8148838: Stream.flatMap(...).spliterator() cannot properly split after tryAdvance()

Please review and sponsor this fix: https://bugs.openjdk.java.net/browse/JDK-8148838 http://cr.openjdk.java.net/~tvaleev/webrev/8148838/r1/ When buffer traversal is already started, and split is requested, then the existing buffer should be carefully transferred to the prefix part. The only problem I see here is the testing time. Due to permuteFunctions() call it increases significantly what I added the flatMap() test. If this becomes an issue, I can write new simple test which tests flatMap() only (without permutations with other intermediate operations). With best regards, Tagir Valeev.