More porting from rep (#2388)

* More porting from rep

* Address review feedback

builtin/logical/mongodb/backend.go

@@ -33,6 +33,8 @@ func Backend() *framework.Backend {
 		},
 
 		Clean: b.ResetSession,
+
+		Invalidate: b.invalidate,
 	}
 
 	return b.Backend
@@ -97,6 +99,13 @@ func (b *backend) ResetSession() {
 	b.session = nil
 }
 
+func (b *backend) invalidate(key string) {
+	switch key {
+	case "config/connection":
+		b.ResetSession()
+	}
+}
+
 // LeaseConfig returns the lease configuration
 func (b *backend) LeaseConfig(s logical.Storage) (*configLease, error) {
 	entry, err := s.Get("config/lease")