RFR: 8372946 - TreeMap sub-map entry spliterator is expensive

[Oli Gillespie]

`TreeMap` sub-maps use the default `IteratorSpliterator` implementation for `TreeMap$EntrySetView` which is slow for some operations, because `EntrySetView.size()` iterates all elements. This is most trivially shown by something like `largeTreeMap.tailMap(0L, false).entrySet().limit(1).count()` taking a long time. This showed up in my application, where it was trivial to mitigate by switching to a for loop, but I think the fix is easy enough.

`keySet()` does not have the same problem, as it provides a custom `Spliterator` implementation which is not `Spliterator.SIZED`, and returns `Long.MAX_VALUE` for `estimateSize()` (which is the recommended approach when the size is expensive to compute). I'm *assuming* this optimization was simply missed for the EntryIterator in the original implementation, but I don't know for sure.

This patch fixes the issue by providing a custom spliterator for `EntrySetView`, which is not SIZED. The implementation is copied almost exactly from the equivalent `KeyIterator` classes in this file (`SubMapKeyIterator`, `DescendingSubMapKeyIterator`). The only difference is in `SubMapEntryIterator.getComparator`, for which I copied the implementation from `TreeMap$EntrySpliterator`.


Basic performance test: `map.tailMap(0L, false).entrySet().stream().limit(1).count()` for a `TreeMap` with `10_000_000` entries.

Before (keySet is fast using `SubMapKeyIterator`, entrySet is slow using `IteratorSpliterator`):

class java.util.TreeMap$KeySet
    .stream().limit(1).count() took 0.046ms
    spliterator = SubMapKeyIterator, estimateSize() = 9223372036854775807
class java.util.TreeMap$AscendingSubMap$AscendingEntrySetView
    .stream().limit(1).count() took 218ms
    spliterator = IteratorSpliterator, estimateSize() = 9999999 


After (entrySet is now fast, using `SubMapEntryIterator`):

class java.util.TreeMap$KeySet
	.stream().limit(1).count() took 0.017ms
	spliterator = SubMapKeyIterator, estimateSize() = 9223372036854775807
class java.util.TreeMap$AscendingSubMap$AscendingEntrySetView
	.stream().limit(1).count() took 0.013ms
	spliterator = SubMapEntryIterator, estimateSize() = 9223372036854775807

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Commit messages:
 - Attempt to fix 8372946

Changes: https://git.openjdk.org/jdk/pull/28608/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28608&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8372946
  Stats: 70 lines in 1 file changed: 68 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28608.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28608/head:pull/28608

PR: https://git.openjdk.org/jdk/pull/28608