Add command to check telegram connection status

[MurzNN]

Add command to check telegram connection status

[tulir]

A basic ping command shouldn't be too hard, but telegram-mtproto doesn't properly throw errors, so it's hard to differentiate between different inactive states (logged out, connection problem, server salt problem, etc..).

[MurzNN]

Maybe we can do all checks via simply sending help command to botfather and check correct reply?

[tulir]

The ping command I added in eaff820 calls contacts.getContacts with a 2.5s timeout to determine if the connection is working. I don't think messaging a bot will help, since it won't give more specific errors. The problem is in telegram-mtproto, so we'll just have to wait for that to get fixed.