perf(retention): improve performance of exemplars removal (#1018)

* Revert "fix: allow cache eviction and write-back while purging storage (#962)"

This reverts commit cad1afc.

* fix(exemplars): optimise exemplars removal

* perf(exemplars): batch insert

* perf(ingestion): adjust storage queue size

* perf(ingestion): make exemplars to handle batches queue during data removal

* fix tests

Author: kolesnikovae

pkg/storage/db.go

@@ -120,7 +120,7 @@ func (s *Storage) newBadger(name string, p prefix, codec cache.Codec) (d *db, er
 		})
 	}
 
-	s.periodicTask(s.badgerGCTaskInterval, func() {
+	s.maintenanceTask(s.badgerGCTaskInterval, func() {
 		diff := calculateDBSize(badgerPath) - d.lastGC
 		if d.lastGC == 0 || s.gcSizeDiff == 0 || diff > s.gcSizeDiff {
 			d.runGC(0.7)