OEP-10613: Forgot to change call to readDataHash() in a previous commit.

Eventually I'd like to deprecate readEditableDataHash() and hopefully
remove it entirely.
OEP-10613: Fixed a dumb bug in retrieving the child key for a nested hash in _setHash().

This is why a previously reverted commit was causing trouble.
OEP-10470: Fixed bug in determining memory location of nested records in an array.

The mechanism by which we check for existing store keys is now simpler and more
robust at the same time.
OEP-10936: Now unlinking data hash in parent store when nested record deleted from nested store.

This was an oversight. If a nested record is deleted in a nested store, its
data hash should be unlinked from its parent's data hash in the parent store.