Use only chat_id

[axtrace]

Is your feature request related to a problem? Please describe.
Anywhere in code I use user_id and chat_id. I remember book file and position by user_id, and send message by chat_id. It looks redundantly

Describe the solution you'd like
Use only chat_id for remembering position and for sending messages

• Check what will happen if user delete chat in his client and some time later join bot again for continue reading. Will it be the same chat_id?

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Now it's possible to upload book via group chat with bot and continue reading it through direct chat. So different