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

postgres/README.md

@@ -17,6 +17,7 @@ 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() *sql.DB
 ```
 ### Installation
 Postgres is tested on the 2 last [Go versions](https://golang.org/dl/) with support for modules. So make sure to initialize one first if you didn't do that yet:

postgres/postgres.go

@@ -189,6 +189,11 @@ func (s *Storage) Close() error {
 	return s.db.Close()
 }
 
+// Return database client
+func (s *Storage) Conn() *sql.DB {
+	return s.db
+}
+
 // gcTicker starts the gc ticker
 func (s *Storage) gcTicker() {
 	ticker := time.NewTicker(s.gcInterval)

postgres/postgres_test.go

@@ -177,3 +177,7 @@ func Test_SslRequiredMode(t *testing.T) {
 func Test_Postgres_Close(t *testing.T) {
 	utils.AssertEqual(t, nil, testStore.Close())
 }
+
+func Test_Postgres_Conn(t *testing.T) {
+	utils.AssertEqual(t, true, testStore.Conn() != nil)
+}