Fix the isc_hp initialization and memory usage

Fix the isc_hp initialization and memory usage

Previously, the isc_hp_init() could not lower the value of
isc__hp_max_threads, but because of a mistake the isc__hp_max_threads
would be set to HP_MAX_THREADS (e.g. 128 threads) thus it would be
always set to 128.  This would result in increased memory usage even
when small number of workers were in use.

Change the default value of isc__hp_max_threads to be 1.

Additionally, enforce the max_hps value in isc_hp_new() to be smaller or
equal to HP_MAX_HPS.  The only user is isc_queue which uses just 1
hazard pointer, so it's only theoretical issue.