[FOLLOW-UP] Additional message processing from chat-relay

[Antreesy]

Missing from chat-relay:

• Fallback polling (to check for missing messages)
  • Once per 5 minutes? / once per 100 messages?
  • Fetch back from the last? forward from the last given by server?

Two bonuses to process:

• Poll sys messages now include additional information
• Thread object is included in thread-created sys message

Originally posted by @DorraJaouad in #16406 (comment)