RFR: Factor out keep-alive barrier

[Aleksey Shipilev]

On 09/06/2017 05:19 PM, Roman Kennke wrote:
> http://cr.openjdk.java.net/~rkennke/keepalivebarrier/webrev.01/

Copy-paste error in description:

 285   diagnostic(bool, ShenandoahKeepAliveBarrier, true,                        \
 286           "Turn on/off SATB barriers in Shenandoah")                        \

Otherwise okay.

Thanks,
-Aleksey