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

mysql/mysql_test.go

@@ -162,3 +162,7 @@ func Test_MYSQL_Non_UTF8(t *testing.T) {
 func Test_MYSQL_Close(t *testing.T) {
 	utils.AssertEqual(t, nil, testStore.Close())
 }
+
+func Test_MYSQL_Conn(t *testing.T) {
+	utils.AssertEqual(t, true, testStore.Conn() != nil)
+}