Admin Train Release via Python Client leaves driver stuck

[BLuedtke]

Procedure using the python cmdline client:

1. driver grabs a train that is on the tracks
2. admin force-releases that train
3. driver tries to grab a new train -> "you can only grab one train!" (<- this is printed because the admin release does not reset the grab id that the python client has saved, thus it thinks the driver has already grabbed a train)
4. driver tries to release their current train -> "System not running!" (<- server rejects, and python client prints out completely incorrect error message in all cases)

There should to be a way to properly reset the grab_id on the python client side, i.e., to synchronize with the server to some degree.

[eyip002]

Related to #23