swarm/storage: fix garbage collector index skew (#18080)

On file access LDBStore's tryAccessIdx() function created a faulty
GC Index Data entry, because not indexing the ikey correctly.
That caused the chunk addresses/hashes to start with '00' and the last
two digits were dropped. => Incorrect chunk address.

Besides the fix, the commit also contains a schema change which will
run the CleanGCIndex() function to clean the GC index from erroneous
entries.

Note: CleanGCIndex() rebuilds the index from scratch which can take
a really-really long time with a huge DB (possibly an hour).

swarm/storage/schema.go

@@ -1,6 +1,17 @@
 package storage
 
+// The DB schema we want to use. The actual/current DB schema might differ
+// until migrations are run.
+const CurrentDbSchema = DbSchemaHalloween
+
+// There was a time when we had no schema at all.
+const DbSchemaNone = ""
+
 // "purity" is the first formal schema of LevelDB we release together with Swarm 0.3.5
 const DbSchemaPurity = "purity"
 
-const CurrentDbSchema = DbSchemaPurity
+// "halloween" is here because we had a screw in the garbage collector index.
+// Because of that we had to rebuild the GC index to get rid of erroneous
+// entries and that takes a long time. This schema is used for bookkeeping,
+// so rebuild index will run just once.
+const DbSchemaHalloween = "halloween"