* Separated usn_bepreop operations from usn_betxnpreop operations.
usn_bepreop_modify and _modrdn add "entryusn: #" to the mods,
which should be handled before the transaction starts.
* Introduced SLAPI_PLUGIN_BE_TXN_PRE_DELETE_TOMBSTONE_FN plugin
hook to modify the tombstone entry at the betxn timing.
* Eliminated preventryusn (SLAPI_ATTR_ENTRYUSN_PREV). It was used
to undo the incremented entryusn when the deletion fails.
Since the operation is now executed in the transaction and it
is aborted if the operation fails, the explicit undo is not
needed in the usn postop.