[FR] Add abstraction for message read status

[blueset]

Implementations by IM platforms

IM	Read by you	Read by others
Telegram	Per chat, up to a message; reversible per chat	Per chat, per message, binary
WeChat	Per chat, binary	None
QQ	Per chat, binary	None
Facebook Messenger	Per chat, binary; reversible	Per chat, per message, per recipient, ternary
...	...	...

Idea

Status object to report to read status per direction, up to the highest granularity. Slave channels can selectively ignore details that it does not support.

No exception should be raised on these statuses, as it is not significant to interrupt normal communication.

Significance up to this point

The current only master channel (ETM) does not support read status at all due to restriction of Telegram Bot API. Slave channels that supports it are dealing it on their own. By far the only use case I can think of is to have [an option in ETM/ a middleware] to switch between mark-as-read strategies:

• Mark on new message from user
• Mark on reception