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

arangodb/README.md

@@ -16,6 +16,7 @@ func (s *Storage) Set(key string, val []byte, exp time.Duration) error
 func (s *Storage) Delete(key string) error
 func (s *Storage) Reset() error
 func (s *Storage) Close() error
+func (s *Storage) Conn() driver.Client
 ```
 ### Installation
 ArangoDB is tested on the 2 last (1.14/1.15) [Go versions](https://golang.org/dl/) with support for modules. So make sure to initialize one first if you didn't do that yet:

arangodb/arangodb.go

@@ -246,3 +246,8 @@ func (s *Storage) gc() {
 		}
 	}
 }
+
+// Return database client
+func (s *Storage) Conn() driver.Client {
+	return s.client
+}

arangodb/arangodb_test.go

@@ -132,3 +132,7 @@ func Test_ARANGODB_Non_UTF8(t *testing.T) {
 func Test_ARANGODB_Close(t *testing.T) {
 	utils.AssertEqual(t, nil, testStore.Close())
 }
+
+func Test_ARANGODB_Conn(t *testing.T) {
+	utils.AssertEqual(t, true, testStore.Conn() != nil)
+}