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_cases_test.go

@@ -5,11 +5,12 @@ package needle_map
 
 import (
 	"fmt"
-	"github.com/seaweedfs/seaweedfs/weed/storage/types"
-	"github.com/stretchr/testify/assert"
 	"log"
 	"os"
 	"testing"
+
+	"github.com/seaweedfs/seaweedfs/weed/storage/types"
+	"github.com/stretchr/testify/assert"
 )
 
 func Test5bytesIndexLoading(t *testing.T) {