always use prepare_commit in ldb transaction commits if possible

always use prepare_commit in ldb transaction commits if possible

The reason we need this is to make multi-tdb transactions safe, with
the partition module. The linked_attributes and repl_meta_data modules
now do extra processing when the transaction ends, and that processing
can fail. When it fails we need to cancel the transaction, which we
can only do if the hook is on the prepare commit instead of the end
transaction call. Otherwise the partition module cannot ensure that no
commit has been done on another partition.