Periodically trigger compaction

This addresses an issue where the compaction triggered on cutting
a new block doesn't find anything as the writers are still active on the
block that should be ready for compaction.

db.go

@@ -187,8 +187,16 @@ func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db
 func (db *DB) run() {
 	defer close(db.donec)
 
+	tick := time.NewTicker(30 * time.Second)
+	defer tick.Stop()
+
 	for {
 		select {
+		case <-tick.C:
+			select {
+			case db.compactc <- struct{}{}:
+			default:
+			}
 		case <-db.compactc:
 			db.metrics.compactionsTriggered.Inc()