dsdb partition.c: Make partition_copy_all aysnc.

dsdb partition.c: Make partition_copy_all aysnc.

partition_copy_all uses ldb_wait to wait for the update to the primary
partition to complete, when updating a special dn.  If a module higher
up the chain inserts a callback, the code blocks in ldb_wait and does
not complete.  This change replaces the ldb_wait logic with a callback.

Currently there is no code that triggers this bug, however the up coming
audit logging changes do trigger this bug.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>