RFR 8171971: Fix timing bug in JVM management of package export lists

[harold seigel]

Hi,

Please review this fix for the package export timing holes discussed in 
JDK-8171971.  The fix reduces the number of PackageEntry fields that are 
used to maintain a package's export state and uses the Module_lock to 
protect all access to these fields.

Also, in cases where a package transitions from having qualified exports 
to being unqualifiedly exported, it fixes the cleanup of its qualified 
export list by removing the _exported_pending_delete field and using 
just is_unqual_exported() to determine when the qualified exports list 
can be purged (at a safepoint).

Open Webrev: http://cr.openjdk.java.net/~hseigel/bug_8171971/webrev/

JBS Bug: https://bugs.openjdk.java.net/browse/JDK-8171971

The fix was tested with the hotspot, java/lang, java/util, java/io, JFR, 
and other JTReg tests, the JCK lang and VM tests, RBT tier2 - tier5 
tests on LinuxX64, and the colocated and non-colocated NSK tests.

Thanks, Harold