New needle_map.CompactMap() implementation for reduced memory usage (#6842)

* Rework `needle_map.CompactMap()` to maximize memory efficiency.

* Use a memory-efficient structure for `CompactMap` needle value entries.

This slightly complicates the code, but makes a **massive** difference
in memory efficiency - preliminary results show a ~30% reduction in
heap usage, with no measurable performance impact otherwise.

* Clean up type for `CompactMap` chunk IDs.

* Add a small comment description for `CompactMap()`.

* Add the old version of `CompactMap()` for comparison purposes.

weed/storage/needle_map/compact_map_perf_test.go

@@ -43,6 +43,7 @@ func TestMemoryUsage(t *testing.T) {
 
 		PrintMemUsage(totalRowCount)
 		now := time.Now()
+		fmt.Printf("\tCompactMap = %s", m.String())
 		fmt.Printf("\tTaken = %v\n", now.Sub(startTime))
 		startTime = now
 	}