Remove extrahandle size from netmgr

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().

Closes: #3227 (closed)