DUK_UNREF() additions for non-mark-and-sweep case

Fix some compile warnings when DUK_OPT_NO_REFERENCE_COUNTING is defined.
Release note update: non-refcount warnings
Merge branch 'msonly-warn-fixes'
Test case for leak-like object entry part behavior
Count used keys when resizing object entry part

The new behavior is slower than the previous one of just using the previous
entry part size as a rough estimate, but avoids undesirable behavior such as:
test-bug-object-prop-alloc-unbounded.js.
Release note: object resize fix
Merge branch 'entrypart-resize-fix'