locking support

[JarekW]

support for doctrine locking (TO DISCUSS):

• read lock (LockMode::PESSIMISTIC_WRITE) from fetch, through validation to save (in case request is to save the form).
  This prevents simultaneous changes within one request.

• If entity have version field, add it to hidden form field and on post-back fetch entity with LockMode::OPTIMISTIC. If versions don't match show form with changed data and message that it was changed by someone else. It could be implemented by developers with current version, but it would require some additional work with each form.
  This prevents simultaneous changes across requests (from opening form to saving it).

Thiese locks are not mutually exclusive, they could (should) be used together.

[szymach]

Proposed solution:

1. Remove public access to DataIndexer in all admin elements, make the elements themselves handle fetching objects, which would allow handling locking.
2. Add an option to Doctrine form elements to put a lock on fetched object regardless of whether the form is submitted or not. This will have an impact on speed, but it is better than having to deal with locks in a makeshift fashion.

[chives]

Another solution could be extending DoctrineDataIndexer with $lockMode constructor argument which could be passed from doctrine admin elements