default json rpc timeout is defined in lib/tfgrid/gateways.v and used everywhere in the tfgrid module

[robvanmieghem]

Is this value only reasonable for gateway calls or does it truly make sense for all tfgrid calls? If only for gateway calls, let's rename it, if not, let's move it to lib/tfgrid/tfgrid_client.v Maybe it makes sense to put it as a field on TFGridClient struct and add a send_json_rpc function to it as well which uses this field value.