Fix FileNotFoundException due to race in NativeFileSorter

[subhramit]

Closes #5765

The bug apparently was a race due to the lazy iterator escaping its DoFn's lifecycle.
The flow goes -

1. SortValues internally wrote spilled data to temp files
2. Beam's SortValuesDoFn deleted those temp files when it finished processing the element (end of @ProcessElement)
3. The KvToTuple map returns a lazy Iterable[(K2, V)] that wrapped the DecodingIterable
4. Downstream List.prependedAll called .iterator on the lazy wrapper after the DoFn has already cleaned up the temp files, causing the FileNotFoundException

List.prependedAll          <- forces the iterator here (lazy consumer)
  -> anon Iterable.iterator
    -> NativeFileSorter.mergeSortedFiles  <- file already deleted

Fix was simply to force full consumption of the DecodingIterable while the DoFn is still alive and the temp files still exist.

Now by the time .map returns a (K1, Iterable[(K2, V)]), the Iterable is a plain Scala Vector with no references to Beam internals or temp files.