repair validation logic for 'stock buffer' and 'default order status'

[jhoule86]

• Fix https://github.com/wayfair-contribs/plentymarkets-plugin/issues/140 by making Stock Buffer and Default Order Status settings optional to the plugin's user.

• Fix https://github.com/wayfair-contribs/plentymarkets-plugin/issues/142 by casting Default Order Status to float instead of int.

• Fix https://github.com/wayfair-contribs/plentymarkets-plugin/issues/143 by checking incoming Default Order Status values against the Plentymarkets system by using OrderStatusRepositoryContract.

• Make the KeyValueRepository's putOrReplace method handle nulls correctly by avoiding saving new entries for null and/or by deleting old entries when someone attempts to set a value to null.

[jhoule86]

going to replace this with a version that displays the order statuses in the UI