s4-drepl: Ensure that the op->source does not get deallocated too early

s4-drepl: Ensure that the op->source does not get deallocated too early

We need to have the struct dreplsrv_partition_source_dsa around until the end of the
async op, so we use talloc_reference after carefully checking the callers and
making the modifications required.

This prevents a crash when replicating partitions in the vampire_dc test after
adding DNS replication at join time.

Andrew Bartlett