RFR: 8202230: Provide accessors for JNIHandles storage objects

[Kim Barrett]

Please review this addition to JNIHandles, providing access to the
OopStorage objects used to manage the global and weak global handles.
The accessors assert the storage objects have been allocated.

Also changed direct uses of the storage object variables to instead
use the new accessors, to get the initialization asserts during
development.

CR:
https://bugs.openjdk.java.net/browse/JDK-8202230

Webrev:
http://cr.openjdk.java.net/~kbarrett/8202230/open.00/

Testing:
{jdk,hs}-tier{1,2,3} for all Oracle platforms.