s4:dsdb Rework schema loading and add schema reloading
This commit reworks Samba4's schema loading code to detect when it
needs to reload the schema. This is done by watching the @REPLCHANGED
special DN.
The reload happens by means of a callback, which is only set when the
schema is loaded from the ldb - not when loaded from an LDIF file or
DRS.
We also rework the global schema handling - instead of storing the
pointer to the global schema in each ldb, we store a flag indicating
that the global schema should be returned at run time. This makes it
much easier to switch to a new global schema.
Andrew Bartlett