The team_migration_started event is sent when a Slack team is about to be
migrated between servers. The websocket connection will close immediately after it is sent.
Occasionally we need to move Slack teams to a new server. To avoid any data
synchronization bugs or race conditions we disconnect all clients from a team
before starting this process. By the time a client has reconnected the process
is usually complete, so the impact is minimal.
When clients receive this event, immediately start a reconnection
process by calling rtm.start again. You may
receive occasional migration_in_progress errors when re-calling rtm.start. If this happens you should wait a few seconds and try again. If the error continues you should wait longer before retrying, and so on.