Simplify Address Sanitizer tweaks in mem.c

Previously, whole isc_mempool_get() and isc_mempool_set() would be replaced by simpler version when run with address sanitizer.

Change the code to limit the fillcount to 1 and freemax to 0. This change will make isc_mempool_get() to always allocate and use a single new item and isc_mempool_put() will always return the item to the allocator.