RFR(s): 8213574: Deadlock in string table expansion when dumping lots of CDS classes

[Robbin Ehn]

Hi all, please review.

The re-sizing operation is run by the ServiceThread (JavaThread). To be 
safepoint polite it pauses the operation and do safepoint checks. Operations on
the hashtable that visit multiple bucket are mutual exclusive. This means you
can't iterate over the table during a safepoint if there is paused resize.

The hashtable uses a Mutex, which means if there were other threads using it
during the safepoint the VM thread sneaking would break it. Since there are no 
such users it is safe to access it without locks in side the safepoint. That is
how rehash works today.

Until we sorted this out better in 8213742,  I'm adding a safepoint scanning
operation that can handle a paused resize and which skips the lock.

CR: https://bugs.openjdk.java.net/browse/JDK-8213574
Webrev: http://cr.openjdk.java.net/~rehn/8213574/webrev/

Passes t1-3 and 8213587 new test which triggered the issue.

Thanks, Robbin