bot, irc, coretasks: more robust bot.connection_registered value

[dgw]

Description

The connection_registered attribute itself is now a property, which first checks to see if the bot's backend exists (is not None) and then whether the backend.is_connected(), before finally seeing if the internal _connection_registered flag has been set.

Also added a few places where the internal flag should be reset back to its initial value of False, such as if the bot is about to quit or the backend's on_close() handler was called.

All of this is to try and make bot.connection_registered reliable for plugins like remind, which have scheduled tasks that might need to send data to IRC and need a reliable way to check whether the connection is ready to accept such data before operating.

Checklist

• I have read CONTRIBUTING.md
• I can and do license this contribution under the EFLv2
• No issues are reported by make qa (runs make quality and make test)
• I have tested the functionality of the things this change touches
  • Note that this required adding a bit more setup to two test_coretasks units.

[Exirel]

That will do.

I wonder if a Threading.Event could be used for _connection_registered instead of a boolean, but that's a nitpick and not necessary right now.

[dgw]

Threading.Event could be used for _connection_registered instead of a boolean

This is a neat idea and worth looking into, largely because of the .wait() method that would make available.