Use isc_mem_reget() to handle the internal active handle cache

Use isc_mem_reget() to handle the internal active handle cache

The netmgr, has an internal cache for freed active handles.  This cache
was allocated using isc_mem_allocate()/isc_mem_free() API because it was
simpler to reallocate the cache when we needed to grow it.  The new
isc_mem_reget() function could be used here reducing the need to use
isc_mem_allocate() API which is tad bit slower than isc_mem_get() API.