[*] migrate maintenance task logic to PL/pgSQL

[0xgouda]

• Add new admin.maintain_tables() SQL function that abstracts the maintenance logic from the Go code
  by handling the fetch of all metric tables, updating them one by one, and deleting entries for dropped tables.

[coveralls]

Pull Request Test Coverage Report for Build 19766198703

Details

• 3 of 4 (75.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.09%) to 73.556%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
internal/sinks/postgres.go	3	4	75.0%

Files with Coverage Reduction	New Missed Lines	%
internal/sinks/postgres.go	1	74.25%

Totals
Change from base Build 19765583712:	-0.09%
Covered Lines:	3616
Relevant Lines:	4916

---

💛 - 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)
	}
}

[0xgouda]

I want go part look like this:

done, please review.

[0xgouda]

Increased test timeouts to 10 minutes because Test_MaintainUniqueSources_DeleteOldPartitions/MaintainUniqueSources now takes more than 2 minutes alone, due to calls to PG_SLEEP(60) in MaintainUniqueSources().