update contexts last_active_at when enter to LIVE state

Resolved issue during stack rollback or extended downtime surpassing 4 hours.
When restarting the stack with concurrent application and haf startup,
 the autodetach mechanism detaches stale contexts (all contexts ar active later than 4h).
This led to exceptions being raised by hive.app_next_block of the applications if their
the last active timestamps were not updated before the autodetach check.