PersistFilter is not reentrant

Issue #597 ('[Persist Extension: UnitOfWork.begin() throws IllegalStateException when called multiple times](https://github.com/google/guice/issues/597)') already mentioned that PersistFilter is not reentrant;

From my Google App Engine application:
- Request comes in
- FilterChain starts
- PersistFilter starts a unit of work
- continues FilterChain
- I need to login, redirects to login url
- Filter Chains runs again
- Persist filter throws exception starting a new unit of work.

"**Work already begun on this thread. Looks like you have called UnitOfWork.begin() twice without a balancing call to end() in between.**"

The issue is closed, but the PersistFilter (Guice 4.1.0) is still not reentrant ...