RFR: Fixed two string dedup issues

[Aleksey Shipilev]

On 10/10/2017 07:35 PM, Zhengyu Gu wrote:
> safepoint test does not tell us if we are evacuating objects in parallel phase or concurrent phase,
> where we have to deal with differently. For shenandoah specific safepoints, we know dedup thread is
> suspended, can safely queue strings.

Ah! So here is the most important bit which deserves to be in the comment!

   // SafepointSynchronize::is_at_safepoint() is not sufficient, because generic safepoints
   // that might happen during concurrent evacuation are not suspending the deduplication thread,
   // and pushes to the dedup queue are unsafe.

-Aleksey