Add a flag to let the update thread to exit when plugin is unloaded

[Sniffleupagus]

The update_value thread does not exit when the plugin is disabled. it sticks around until pwnagotchi exits.

Description

I added a boolean "keep_going = True" to the PisugarServer. In update_value, I changed "while true:" to "while keep_going:". Then in on_unload, keep_going it set False. The background thread will exit the next time it starts the loop.

Motivation and Context

Threads would accumulate when pisugarx is disabled/enabled.

• [ X ] I have raised an issue to propose this change (required)

How Has This Been Tested?

Tested on pwnagotchi 2.9.5-3.

Types of changes

• [ X ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [ X ] My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [ X ] I've read the CONTRIBUTION guide
• [ X ] I have signed-off my commits with git commit -s