Database version in features (#5627)

* adding database.version in the serverInfo (only MongoDB, it gives undefined when using Postgres)

* . correction of old 'features' tests
. adding engine and database in the StorageAdapter interface and implementations

* . version retrieval done in performInitialization
. PostgreSQL version

* performInitialization now returns a Promise

src/Adapters/Storage/Mongo/MongoStorageAdapter.js

@@ -126,6 +126,8 @@ export class MongoStorageAdapter implements StorageAdapter {
   client: MongoClient;
   _maxTimeMS: ?number;
   canSortOnJoinTables: boolean;
+  databaseVersion: string;
+  engine: string;
 
   constructor({
     uri = defaults.DefaultMongoURI,
@@ -136,6 +138,7 @@ export class MongoStorageAdapter implements StorageAdapter {
     this._collectionPrefix = collectionPrefix;
     this._mongoOptions = mongoOptions;
     this._mongoOptions.useNewUrlParser = true;
+    this.engine = 'MongoDB';
 
     // MaxTimeMS is not a global MongoDB client option, it is applied per operation.
     this._maxTimeMS = mongoOptions.maxTimeMS;
@@ -959,7 +962,15 @@ export class MongoStorageAdapter implements StorageAdapter {
   }
 
   performInitialization(): Promise<void> {
-    return Promise.resolve();
+    // databaseVersion
+    return this.connect()
+      .then(() => {
+        const adminDb = this.database.admin();
+        return adminDb.serverStatus();
+      })
+      .then(status => {
+        this.databaseVersion = status.version;
+      });
   }
 
   createIndex(className: string, index: any) {