Properly disable the "water" in isc_mem

The proper way how to disable the water limit in the isc_mem context is
to call:

    isc_mem_setwater(ctx, NULL, NULL, 0, 0);

this ensures that the old water callback is called with ISC_MEM_LOWATER
if the callback was called with ISC_MEM_HIWATER before.

Historically, there were some places where the limits were disabled by
calling:

    isc_mem_setwater(ctx, water, water_arg, 0, 0);

which would also call the old callback, but it also causes the water_t
to be allocated and extra check to be executed because water callback is
not NULL.

This commits unifies the calls to disable water to the preferred form.

lib/dns/cache.c

@@ -950,7 +950,7 @@ dns_cache_setcachesize(dns_cache_t *cache, size_t size) {
 		/*
 		 * Disable cache memory limiting.
 		 */
-		isc_mem_setwater(cache->mctx, water, cache, 0, 0);
+		isc_mem_setwater(cache->mctx, NULL, NULL, 0, 0);
 	} else {
 		/*
 		 * Establish new cache memory limits (either for the first