deprecate and remove Persistit Data Backend (PDB)

[Kortanul]

ForgeRock introduced the Persistit Data Backend (PDB) in OpenDJ 3.0.0 and deprecated it in DS 5 (i.e. OpenDJ 4.0.0).

There are multiple reasons why they abandoned it:

• ForgeRock had to fork the project because Akiban was acquired and no longer maintains it.
• Even after patches from FR, it has remaining issues that can cause deadlock in production,
• Unit tests for Persistit are painfully slow and fail if you actually run them.
• PDB is no longer needed because JE licensing is now compatible with OEM deployments of OpenDJ / Wren:DS. We still need to upgrade the JE backend – see #1.

[siepkes]

Seems like a good idea; Less maintenance burden for us.