RFR(S): 8228631: Fix inconsistent OopStorage::Block owner usage

[Kim Barrett]

Please review this cleanup of the use of the _owner member of
OopStorage blocks.  It's now just the intptr_t address of the owning
storage object, and no longer sometimes used as the actual owning
object.

CR:
https://bugs.openjdk.java.net/browse/JDK-8228631

Webrev:
http://cr.openjdk.java.net/~kbarrett/8228631/open.00/

Testing:
mach5 tier1