[*] migrate maintenance task logic to PL/pgSQL

[0xgouda]

• Create new PL/pgSQL functions containing the maintenance logic.
  • admin.try_get_maintenance_lock().
  • admin.update_listing_table(metric_table_name text).
  • admin.remove_dropped_tables_listing(existing_metrics text[]).
• Call these functions from the Golang maintenance routine.

[coveralls]

Pull Request Test Coverage Report for Build 19699862818

Details

• 18 of 22 (81.82%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.09%) to 73.597%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/sinks/postgres.go	18	22	81.82%

Totals
Change from base Build 19665354941:	-0.09%
Covered Lines:	3646
Relevant Lines:	4954

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💛 - Coveralls

[pashagolub]

I want go part look like this:

func (pgw *PostgresWriter) MaintainUniqueSources() {
	sql := "SELECT admin.maintain_tables() WHERE pg_try_advisory_lock(1571543679778230000)"
	logger := log.GetLogger(pgw.ctx)
	logger.Info("Starting maintainence...")
	if _, err := pgw.sinkDb.Exec(pgw.ctx, sql); err != nil {
		logger.Error("Maintaining measurement tables failed:", err)
	}
}