RFR: Factor out keep-alive barrier

[Roman Kennke]

Am 06.09.2017 um 17:28 schrieb 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.

Pushed with this change and null-check re-instated in jniHandles.cpp

Roman