Add chef_wm_base:maybe_process_client/2

Open Source Chef Server and Private Chef Server process clients a
little bit differently.  This seems like the least invasive way to
handle it, and it fits with the pattern we've taken with similar
issues in the past.
Clients are actors
maybe_process_client/2 => object_creation_hook/2

More general name
Merge branch 'cm/OC-7440'