fix (sftp): azure connection timeout

Azure SFTP only hold the connection for 2 minutes .... So we got to
flush our cache before it occurs, otherwise we end up with a "Connection lost" error

server/plugin/plg_backend_sftp/index.go

@@ -24,7 +24,7 @@ type Sftp struct {
 func init() {
 	Backend.Register("sftp", Sftp{})
 
-	SftpCache = NewAppCache()
+	SftpCache = NewAppCache(1, 1)
 	SftpCache.OnEvict(func(key string, value interface{}) {
 		c := value.(*Sftp)
 		if c == nil {