setGlobalDownloadLimit and setGlobalUploadLimit will now modify the
alternative speed limits if they are currently enabled and the regular
speed limits otherwise.
Add also two new commands to toggle the state of the alternative speed
limits and get their current state.
Closes#2203.
New GET methods:
* version/api: current API version
* version/api_min: min API version supported
The API version is an integer and it's returned as plain text.
Whenever the API is changed, the api version is increased.
Whenever the API is changed in a way that is not backward compatible,
api_min is increased.