tdb2: open hook for implementing TDB_CLEAR_IF_FIRST

tdb2: open hook for implementing TDB_CLEAR_IF_FIRST

This allows the caller to implement clear-if-first semantics as per
TDB1.  The flag was removed for good reasons: performance and
unreliability, but SAMBA3 still uses it widely, so this allows them to
reimplement it themselves.

(There is no way to do it without help like this from tdb2, since it has
 to be done under the open lock).