Split to TFCALL and TFCALLASYNC 

Currently there is the `TFCall` command with an `_async` variable that determines whether the `TFCALL` or `TFCALLASYNC` command is sent.
We need to decide whether to leave the situation as it is, or add two separate commands: `TFCall` and `TFCallAsync` that will call the current `TFCall` (which will become private) and set the appropriate value in `_async`.