Flows: Integrate SQLX repository for persistent flow event store

[zaychenko-sergei]

Following the results of #578 , make a similar change for main flow event store:

• implement shared test package and thoroughly cover flow event store operation with tests, cleanup using in-memory version
• implement Postgres version of the flow events repository and stabilize it with the shared test package
• implement SQLite version of the flow events repository and stabilize it with the shared test package