SAHashMap isn't quite a port of HashMap to StructuredArray. While
HashMap overflows to a red–black tree SAHashMap does not. This makes
performance comparisons problematic since different code is tested.

Directly porting HashMap to StructuredArray is not possible since the
table in HashMap is heterogeneous but needs to be homogeneous for
StructuredArray. The compromise implemented in this change is to store
the red–black tree in the next node instead of the node in the table.

This commit includes the following changes:

- sync code based on JDK 8u92, also includes indentation
- store a red–black tree in the next node if there are too many
  overflows
- remove the sentinel flag on Node, use a sentinel object in the key
  due to padding this should save two words per entry and bring memory
  consumption in line with HashMap
  this may help performance
- HashIterator, KeySpliterator, ValueSpliterator, EntrySpliterator were
  missing a few sentinel checks
- SAAbstractMap seems kinda pointless but was left it in and synched

Unfortunately the code could use more testing in the areas of tree
corner cases and streams.