Improve the sessions mechanism (let them be persistent)

[Aloike]

Whenever a query is issued, a session is opened, the query is processed, then the session is closed.

This leads to a large sessions count on the Freebox and there seems to be session collisions when using multiple 'exec' inputs on Telegraf.

• Let the session be persistent.
• Implement a mechanism to renew the session whenever it's disconnected.

[Aloike]

See https://dev.freebox.fr/sdk/os/login/ :

• app_token vs auth_token
• The whole auth/login procedure is described here.