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

[Tagir F. Valeev]

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.