hp implementation requires an object for each thread accessing a hazard pointer. previous implementation had a hardcoded HP_MAX_THREAD value of 128, which failed on machines with lots of CPU cores (named uses 3n threads). We make isc__hp_max_threads configurable at startup, with the value set to 4*named_g_cpus. It's also important for this value not to be too big as we do linear searches on a list.

Closes #1493 (closed)