V2: Move all data gatherers into a plugin system.

[easytarget]

V2 architecture:

• overwatch.py main loop only services hardware and the data dictionary, it does handle any senses or work with sensed data directly.
  • performs startup, config, log setup, schedule init and running, signal handling and all other 'housekeeping' tasks
  • provides gpio, I2C (and SPI) bus objects for the gatherers to use.
  • directly handles all the button pin functions
  • spawns rrdb, http, display and alert tasks/processes that drive the core functionality
    • provides the global data{} dictionary
    • uses queues for display and alerts by extending the dictionary methods for data{}
• gatherers focus on a single dataset, currently: 'system', 'environment', 'connectivity', 'pin'
  • init
  • updater
  • handler for http (show_panel)
  • Attributes (optional) for rrdb (limits, formats, graph)
  • Attributes (optional) for display
  • log directly
• alerters
  • TBD: a plugin system for alerting
  • run in a separate process, possibly demonised so it can alert if the main process dies..
  • Pass the data{} dict to it via a Queue, same as for the display sub-process
  • Allow it to pass data back to the Web interface and display can show alerts?

In General:

• We currently have tables defined in the http, rrd and display classes that should be moved out and become attributes provided by the relevant data gatherers.
• the http server needs to become 'data neutral' as well, gatherers need to provide their own generators (emitting a formatted http table header and rows) so that the httpserver.py module can become data neutral.