Wrong defaults are used for bucket memory watermarks

Issue has been seen on master.

ns_server will sometimes initialize KV buckets with mem_low_wat=2147483647;mem_high_wat=2147483647. I'm not sure when that happens but it has been observed on macOS with cluster_connect -n 1 -T n0:kv -s 512.

The cause seems to be: https://review.couchbase.org/c/ns_server/+/208705/8/apps/ns_server/src/ns_bucket.erl#557

Where defaults of MAX_32BIT_UNSIGNED_INT are used. MAX_32BIT_UNSIGNED_INT is surprisingly defined to be the maximum 32-bit signed integer value.

The memcached configuration.json defaults these values to the maximum value of a size_t, which is a 64-bit unsigned integer. When that value is specified, KV will default to the typical values.

As a result of the incorrect value being specified, both LWM and HWN are set to 2 GiB, which may be above the bucket quota.