RFR: 8015318: Extend Collector with 'finish' operation

[Henry Jen]

Hi,

Please review the webrev at
http://cr.openjdk.java.net/~henryjen/ccc/8015318.0/webrev/

This webrev refactor java.util.Collector interface to allow an
intermediate type used for accumulation and then produce final result
type with a 'finished' function.

Collectors are updated to reflect the changes, and quite a few API changes,

toStringJoiner -> joining
Return type becoming Optional<T> when appropriate
sumBy -> a set of summing[Int,Long,Double]
average[Int,Long,Double]

I'll put specdiff up later for those interested in API changes.

Cheers,
Henry