A couple of performance improvements

o Created and uses PrimitiveLongIterator for getPropertyKeys(). Should be
  used for more as well.
o Use individual statement operation parts to remove the tiny
  StatementOperations delegation at the top.
o TxState#hasChanges() is cheaper
o StateHandlingStatementOperations#getPropertyKeys and getProperty
  just delegates if there's no tx changes.
Merge pull request #934 from tinwelint/2.0-minor-perf-improvements

A couple of performance improvements. co-author @lassewesth, merging.