Replaces MongoDB support with PostgreSQL (#151)

- All schemas have been rewritten to a relational model
- All services have been rewritten to accommodate the new data model
- Adds idempotency keys to core endpoints allowing you to retry requests with no additional side effects
- Adds staged jobs to avoid putting jobs in the queue when transactions abort
- Adds atomic transactions to all methods with access to the data layer

Co-authored-by: Oliver Windall Juhl <oliver@mrbltech.com>

packages/medusa-plugin-contentful/src/subscribers/contentful.js

@@ -1,5 +1,12 @@
 class ContentfulSubscriber {
-  constructor({ contentfulService, eventBusService }) {
+  constructor({
+    contentfulService,
+    productVariantService,
+    productService,
+    eventBusService,
+  }) {
+    this.productVariantService_ = productVariantService
+    this.productService_ = productService
     this.contentfulService_ = contentfulService
     this.eventBus_ = eventBusService