History service: improve a new event creation mechanism

[dmitrii-beliakov]

Investigate how to create new events more efficiently:

• create an event automatically when the first action is going to be saved (improvement for usages, less code on the user's side)
• don't try to create events if there is an existing event for this correlation ID (improvement for chained events)
• (?) create events in async way together with actions (allow to save actions first and then the event)
• create a better approach than currently used when there are multiple events created: ix_ha_flow_event_task_id will not allow to create it. Probably it is better not to write anything at all if there is an entry with the same ID.