Flows: support Flow History feature

[zaychenko-sergei]

1. The history of flow executions and their statuses stays in the flow event store.

2. Model flow events:

   • FlowEventInitiated - starting event
   • FlowEventWaiting - a flow is waiting for some start condition to be satisfied
   • FlowEventQueued - a flow has been initiated in some point in future not earlier than X:
   • FlowEventTaskChanged- a task associated with a flow has change it's status (Queued, Running, Finished)
   • FlowEventAborted - flow was cancelled by the user or aborted by the system.

3. Events should know queuing time, scheduling time, execution start/finish time

4. Flow events should store reasons of their initiation (provenance):

   • Reference another flow's events
   • Reference scheduling rule
   • Indicate initiative by particular user
   • Multiple reasons possible (i.e. 2 or more updates trigger on the inputs of a derived dataset, while it’s still in the queued state)

5. Implement GraphQL API to access the history feed

[zaychenko-sergei]

Note: only GraphQL part remaining after v0.149.0