e2e: improve server

[ernado]

Current go Telegram server lacks lots of functions and is not suitable for sophisticated testing.

I'm looking into features like:

• [ ] Session state/key storage
• [ ] Correct key exchange implementation (https://github.com/gotd/td/commit/a4296aa202ff028f4c08518068913c6564927d66)
• [x] Uploads (#237)
• [x] Downloads (#237)
• [ ] Personal messages
• [ ] Multiple Data Centers
• [ ] CDN

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.