Node config defaults: fix minimum store size bug

The storeSize configuration option accepted values down to 10M, but the node would fail to start when the store size was less than 32MiB.

This commit consolidates the knowledge on minimum store sizes to constants in Node, raising the minimum datastore size to 32MiB in the configuration options.

While the minimum is still "totally arbitrary", as one apt comment mentions, it now at least is applied consistently.