Clean-up action support and check of security_token for webhook

[kozalosev]

Two small enhancements:

• an ability to execute a custom cleanup action before closing the loop (I use it to cancel scheduled async tasks and get rid of the error in the log)
• use of the security_token parameter of the setWebhook method and the X-Telegram-Bot-Api-Secret-Token header in updates to check if the request was actually sent by a Telegram server (no changes are required in client code, since we generate UUID if this parameter wasn't set explicitly)

[szastupov]

Thank you!