cleaning the rtm code, upgrading to the latest version, rtm event han…

[Meherdeep]

• Updated RTC v5.1.0
• Cleaning the session controller
• RTM client and channel event handlers

[maxxfrazer]

LGTM. Thanks @Meherdeep for your hard work getting this PR ready!

[tadaspetra]

Looks good.

Suggestion for the future: I like how the functions are separated into the appropriate files, but the controllers folder maybe isn't a good name for it. The two controllers are rtm_controller.dart and session_controller.dart the other ones are helper files. They still work for the controllers so it doesn't really matter but could be nice to add some more folders to keep things organized.

[Meherdeep]

@tadaspetra I completely agree with your point. The only reason why it is still named session_controller is that it not only just holds the RTC operations but it has some generic methods as well which includes both RTC and RTM (and some UI methods too). But probably I can try creating 3 controller files (maybe in a different folder) where session_controller is the main controller file and then we can have extended rtc_controller and rtm_controller files for their individual methods.