Make the gateway CreateStorage async

[danielmarbach]

When the storage creation requires async initialization the calling code has to either have the ability to use a sync API or needs to do sync over async. Example from RavenDB

            var ravenGatewayDeduplicationConfiguration = new RavenGatewayDeduplicationConfiguration((builder, _) => 
            {
                databaseName = Guid.NewGuid().ToString();
                var documentStore = GetInitializedDocumentStore(databaseName);

                documentStore.Maintenance.Server.Send(new CreateDatabaseOperation(new DatabaseRecord(databaseName)));

                return documentStore;
            })
            {
                EnableClusterWideTransactions = true
            };

CreateStorage should allow async storage creation