UID generator can create DB deadlock

The current UIDGeneratorService relies on hibernate to manage sequence.

Currently the hibernate calls are run inside the global transaction.

As a side effect, when running a bulk import that uses batching and multithreading it does create deadlocks on the tables used to manage the hibernate sequences.

To make this better :

• Step 1 : run the sequence increment in a separated thread/Tx
• Step 2 - move away from hibernate