Change ancient chain segments from root ancient to sub folders

[huyngopt1994]

Reference ethereum/go-ethereum#25487 .

Before The chain segment should be stored in <chaindata/ancient>. Now it will locate in <chaindata/ancient/chain> in case u start the new disk, otherwise support backward compatibility

Handling main idea is here.

	if !common.FileExist(freezer) {
		if !common.FileExist(ancient) {
			// The entire ancient store is not initialized, still use the sub
			// folder for initialization.
		} else {
			// Ancient root is already initialized, then we hold the assumption
			// that chain freezer is also initialized and located in root folder.
			// In this case fallback to legacy location.
			freezer = ancient
			log.Info("Found legacy ancient chain path", "location", ancient)
		}
	}