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/compact_map.go

@@ -175,9 +175,14 @@ func (cs *CompactMapSegment) get(key types.NeedleId) (*CompactNeedleValue, bool)
 // delete deletes a map entry by key. Returns the entries' previous Size, if available.
 func (cs *CompactMapSegment) delete(key types.NeedleId) types.Size {
 	if i, found := cs.bsearchKey(key); found {
-		if cs.list[i].size > 0 && cs.list[i].size.IsValid() {
+		if !cs.list[i].size.IsDeleted() {
 			ret := cs.list[i].size
-			cs.list[i].size = -cs.list[i].size
+			if cs.list[i].size == 0 {
+				// size=0 needles can't be marked deleted by negating, use tombstone
+				cs.list[i].size = types.TombstoneFileSize
+			} else {
+				cs.list[i].size = -cs.list[i].size
+			}
 			return ret
 		}
 	}