schema: Allow schemaUpdateNow to refresh schema during a transaction

schema: Allow schemaUpdateNow to refresh schema during a transaction

When we upgrade a schema from 2008R2 to 2012R2, we want to apply all the
changes in a single transaction - if we can't apply all the updates then
we don't want to be left with a schema halfway in between the two.

However, as we apply each LDIF update, we also want to refresh the
schema. There are 2 reasons for this:
1. The adprep .LDIF files provided by Microsoft have some writes to
schemaUpdateNow in them.
2. Microsoft uses attribute OIDs in their adprep .LDIF files, which
Samba doesn't handle so well. However, we can replace the OIDs with the
attribute's ldapDisplayName and they work fine. But to do this, we need
to query the schema to map the OID to attribute name. And to query the
schema successfully, the schema needs to be refreshed after the new
attribute object has been added.

Basically this patch avoids bailing out during the dsdb_schema_refresh()
if we are writing schemaUpdateNow as part of a larger transaction.

Pair-programmed-with: Garming Sam <garming@catalyst.net.nz>

Signed-off-by: Tim Beale <timbeale@catalyst.net.nz>
Signed-off-by: Garming Sam <garming@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>