RFR: Make Traversal share code with conc-mark

[Aleksey Shipilev]

On 02/22/2018 12:05 AM, Roman Kennke wrote:
> http://cr.openjdk.java.net/~rkennke/traversal-share-concmark/webrev.00/

*) I think ShConcMark and ShTraversalGC should just be friends, instead of extending the public API
for ShConcMark

Otherwise looks good.

-Aleksey