avoid re-get block when storing the bytes

[iwanbk]

s3-cas/src/cas/fs.rs

Lines 276 to 277 in 19f9841

	// first load the block again from the DB
	let block = match block_map.get_block_obj(&block_hash) {

in above lines, it re-get the data that previously stored, it means extra unnecessary operations.
We could avoid it, the alternative is for the self.meta_store.write_meta_for_block to also returns the data blocks.

See also whether we could use update_and_fetch inside the write_meta_for_block for this purpose