RFR(XS): 8152185: ReferencePendingListLocker incorrectly assumes that the lock is never taken recursively

[Per Liden]

Summary: The ReferencePendingListLocker asserts that the pending list 
lock is never taken recursively. This is incorrect, as it can be taken 
by a GC operation initiated by the ReferenceHandler thread, which might 
already own the lock. The asserts were added as part of JDK-8151601 in 
an attempt to strengthen the code, but they are incorrect and should be 
removed.

Bug: https://bugs.openjdk.java.net/browse/JDK-8152185
Webrev: http://cr.openjdk.java.net/~pliden/8152185/webrev.0/

thanks,
Per