Nuqql-based

Changes:
* Switch to asyncio and increase required Python version to >= 3.7
* Remove copy of the buddy list from Account and rename callback
`UPDATE_BUDDIES` to `GET_BUDDIES` to match new behaviour
* Add callbacks:
* `HELP_WELCOME`: welcome/help message for clients
* `HELP_ACCOUNT_ADD`: help text for adding accounts
* Add configuration option for filtering own messages

Changes:
* Use only one log file
* Add "push-accounts" to configuration/command line arguments
* Add more info messages and extend output of "help" command
* Change callback parameter from account id to Account
* Add tests
* Cleanups and fixes

Changes:
* First release.