Fix gc tool never compacts keys with hash-prefix

[polyrabbit]

GC tool hard-coded chunk.Config fields with:

chunkConf := chunk.Config{
	BlockSize:  format.BlockSize * 1024,
	Compress:   format.Compression,
	GetTimeout: time.Second * 60,
	PutTimeout: time.Second * 60,
	MaxUpload:  20,
	BufferSize: 300 << 20,
	CacheDir:   "memory",
}

This is error-prone - it may miss some fields from format added in the future. One such case is HashPrefix, where user sets it in the format but the gc tool doesnot pass it to chunkConf, which causes the compact sub-command never works.