frontend stuff moved into a database structure, essentially to allow overlays to be processed before database selection; passes all tests (ITS#3080)

slightly rework user/operational attributes handling (including fixing a bug in the logic of the previous change to backend_operational()); cleanup; more improvements to slapo-rwm and back-relay

cleanup

(slapi_x_XXX is still reserved for exported functions that are not part
of the SLAPI specification)

case in extended_op

1) simultaneous operation of multiple active sync replication threads
2) cookie management for individual sync replication thread
   (include rid=%3d to the slapd cookie command line option (-c))