Previously, it was possible to assign a bit of memory space in the
nmhandle to store the client data. This was complicated and prevents
further refactoring of isc_nmhandle_t caching (future work).
Instead of caching the data in the nmhandle, allocate the hot-path
ns_client_t objects from per-thread clientmgr memory context and just
assign it to the isc_nmhandle_t via isc_nmhandle_set().
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