Fix uncleanable size=0 orphans with volume.fsck -forcePurging (#7783)

This is a follow-up fix to PR #7332 which partially addressed the issue.

The problem is that size=0 needles are in a gray area:
- IsValid() returns false for size=0 (because size must be > 0)
- IsDeleted() returns false for size=0 (because size must be < 0 or == TombstoneFileSize)

PR #7332 only fixed 2 places, but several other places still had the same bug:

1. needle_map_memory.go:doLoading - line 43 still used oldSize.IsValid()
2. needle_map_memory.go:DoOffsetLoading - used during vacuum and incremental loading
3. needle_map_leveldb.go:generateLevelDbFile - used when generating LevelDB needle maps
4. needle_map_leveldb.go:DoOffsetLoading - used during incremental loading for LevelDB
5. needle_map/compact_map.go:delete - couldn't delete size=0 entries because:
   - The condition 'size > 0' failed for size=0
   - Even if it passed, negating 0 gives 0 (not marking as deleted)

Changes:
- Changed size.IsValid() to !size.IsDeleted() in doLoading and DoOffsetLoading functions
- Fixed compact_map delete to use TombstoneFileSize for size=0 entries

Fixes #7293

weed/storage/needle_map_memory.go

@@ -40,7 +40,7 @@ func doLoading(file *os.File, nm *NeedleMap) (*NeedleMap, error) {
 			nm.FileCounter++
 			nm.FileByteCounter = nm.FileByteCounter + uint64(size)
 			oldOffset, oldSize := nm.m.Set(NeedleId(key), offset, size)
-			if !oldOffset.IsZero() && oldSize.IsValid() {
+			if !oldOffset.IsZero() && !oldSize.IsDeleted() {
 				nm.DeletionCounter++
 				nm.DeletionByteCounter = nm.DeletionByteCounter + uint64(oldSize)
 			}
@@ -112,10 +112,10 @@ func (nm *NeedleMap) DoOffsetLoading(v *Volume, indexFile *os.File, startFrom ui
 	e := idx.WalkIndexFile(indexFile, startFrom, func(key NeedleId, offset Offset, size Size) error {
 		nm.MaybeSetMaxFileKey(key)
 		nm.FileCounter++
-		if !offset.IsZero() && size.IsValid() {
+		if !offset.IsZero() && !size.IsDeleted() {
 			nm.FileByteCounter = nm.FileByteCounter + uint64(size)
 			oldOffset, oldSize := nm.m.Set(NeedleId(key), offset, size)
-			if !oldOffset.IsZero() && oldSize.IsValid() {
+			if !oldOffset.IsZero() && !oldSize.IsDeleted() {
 				nm.DeletionCounter++
 				nm.DeletionByteCounter = nm.DeletionByteCounter + uint64(oldSize)
 			}