Remove extra copies and stray members from ns_client_t

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.