RFR: 8280915: Better parallelization for AbstractSpliterator and IteratorSpliterator when size is unknown [v5]

[Tagir F.Valeev]

> See the bug description for details.
> 
> I propose a simple solution. Let's allow ArraySpliterator to be non-SIZED and report artificial estimatedSize(), much bigger than the real one. This will allow AbstractSpliterator and IteratorSpliterator to produce prefix whose size is comparable to Long.MAX_VALUE (say, starting with Long.MAX_VALUE/2), and this will enable further splitting of the prefix. This change will drastically improve parallel streaming for affected streams of size <= 1024 and significantly improve for streams of size 1025..20000. The cost is higher-grained splitting for huge streams of unknown size. This might add a minor overhead for such scenarios which, I believe, is completely tolerable.
> 
> No public API changes are necessary, sequential processing should not be affected, except an extra field in ArraySpliterator which increases a footprint by 8 bytes.
> 
> I added a simple test using an artificial collector to ensure that at least two non-empty parts are created when parallelizing Stream.iterate source. More testing ideas are welcome.

Tagir F. Valeev has updated the pull request incrementally with two additional commits since the last revision:

 - Update copyright year
 - Cosmetic fixes

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/7279/files
  - new: https://git.openjdk.java.net/jdk/pull/7279/files/fbe8a704..5306e212

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=7279&range=04
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=7279&range=03-04

  Stats: 4 lines in 3 files changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7279.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7279/head:pull/7279

PR: https://git.openjdk.java.net/jdk/pull/7279