dsdb: ensure we take out a read lock during the dsdb_init

dsdb: ensure we take out a read lock during the dsdb_init

We have to also take it out in the partitions code when we load the
partition backends.

This ensures that the init handlers hold a whole-db lock just as the
search code does.

To ensure the locking count in schema_load is balanced, the
private data is now created in the first lock_read() call.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13379

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>