r3357: removed the need to use TDB_CLEAR_IF_FIRST in Samba4.

r3357: removed the need to use TDB_CLEAR_IF_FIRST in Samba4.

We found a few months ago that TDB_CLEAR_IF_FIRST is extremely
inefficient for large numbers of connections, due to a fundamental
limitation in the way posix byte range locking is implemented. Rather
than the nasty workaround we had for Samba3, we now have a single
"cleanup tmp files" function that runs when smbd starts. That deletes
the tmp tdbs, so TDB_CLEAR_IF_FIRST is not needed at all.