RFR: 8234009: ZGC: Move resurrection unblock to before the _unload.purge() call

[Stefan Karlsson]

Hi all,

Please review this patch to move the resurrection to before the 
_unload.purge() call.

https://cr.openjdk.java.net/~stefank/8234009/webrev.01/
https://bugs.openjdk.java.net/browse/JDK-8234009

After JDK-8230661 we are guaranteed that no mutator holds a weak oop 
containing a "dead" object when the ZResurrection::unblock() call happens.

Therefore, it doesn't matter if the load barrier that the thread 
executes runs the code guarded by ZResurrection::is_blocked() or the 
"non-blocked" part, when ZResurrection::unblock() changes the state. As 
long as the ZResurrection::unblock() call happens after the handshake, 
we are good to go.

Today, we perform the purging and deletion of metadata in 
_unload.purge(), and call ZResurrection:unblock() after that. There's no 
need to delay the unblocking to after the purge. We have the opportunity 
to shrink the resurrection-blocked window by moving the 
ZResurrection::unblock() call to before the call to _unload.purge(), but 
still after the handshake.

Thanks,
StefanK