ModifiableSecurityRealm#getRealmIdentityIterator() is a bit wrong

After the fixes for iterator closing which pertain mainly to the file system realm, we have a new interface CloseableIterator which adds a close() method which throws IOException. However this doesn't fit the general mold for identity handling, and only the FS realm would ever throw IOException (others would throw SQLException or NamingException for example).

Fix this in a couple ways:

1. Drop CloseableIterator and replace it with a ModifiableRealmIdentityIterator which extends Iterator<ModifiableRealmIdentity> and AutoCloseable
2. Override close() to throw RealmUnavailableException

Then make the associated changes to the realm(s).