Add Conn() support to all storage drivers. (#451)

* Add DB() support for Redis driver

* Added support for DB() to all drivers

* Fixed typo in README and Lint issue

* Fix lint issue with ristretto db

* Fix lint issue with bbolt db

* Rename DB() to Conn()

* Replace all instances of _DB with _Conn

* Update all the README files

* Return ArangoDB Client instead of DB

dynamodb/dynamodb.go

@@ -264,3 +264,8 @@ func returnAWSConfig(cfg Config) (aws.Config, error) {
 		}),
 	)
 }
+
+// Return database client
+func (s *Storage) Conn() *awsdynamodb.Client {
+	return s.db
+}