Remove extra copies and stray members from ns_client_t
The ns_client_t is always attached to ns_clientmgr_t which has associated memory context, server context, task and threadid. Use those directly from the ns_clientmgr_t instead of attaching it to an extra copy in ns_client_t to make the ns_client_t more sleek and lean.
Additionally, remove some stray ns_client_t struct members that were not used anywhere.