s4:dsdb Rework partitions module for better tracing

s4:dsdb Rework partitions module for better tracing

This means we need to create a fake 'module' which only has a 'next'
pointer, so that we can now ldb_next_request() (which incorporates
tracing).  The remainaing stub of partition_request() is retained so
that we can indicate which partition an operation is destined for.

Similar tracing is added to the transaction handlers.

Andrew Bartlett