Remove inactivereqs from isc__nmsocket_t
The .inactivereqs was acting as locked memory pool because the uvreq was always completely initialized in isc___nm_uvreq_get(). With the introduction of better memory allocator, this is not needed any more.
Additionally, pushing and popping the uvreq from the arraystack hides use-after-use errors.
Closes #3166 (closed)