tsdb: reduce sleep time when reading WAL (#10859)

The code sleeps for a short time to allow goroutines to finish, however
it seems the duration can be reduced a lot, speeding up the reading
process.

I checked using some WAL data from production, and the queue is almost
always empty at the time we enter `waitForIdle()` so there is no danger
of spinning in the tight loop.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/head_wal.go

@@ -438,7 +438,7 @@ func (wp *walSubsetProcessor) waitUntilIdle() {
 	}
 	wp.input <- []record.RefSample{}
 	for len(wp.input) != 0 {
-		time.Sleep(1 * time.Millisecond)
+		time.Sleep(10 * time.Microsecond)
 		select {
 		case <-wp.output: // Allow output side to drain to avoid deadlock.
 		default: