Queries and Extents with non-TX-read will not work in a multi-session setting

[tzaeschke]

Queries and extents may fail or return wrong results when used outside of a transaction with the non-transactional-read feature. The reason is that they may rely on iterators, which may be modified by concurrently by other transactions.

Solutions:

1. Disallow Extents outside of transactions. JDO spec 3.1, section 5.6 only requires Queries and navigation to work with non-TX-read
2. For queries, use infinite batch size, i.e. load all results, at least as HOLLOW. We should consider putting a warning out if the result is too big.