Change the metadata DB

[iwanbk]

Sled has some issues (see #1)

• hide the sled::Error and sled::Tree from the outside cas/fs.rs refactor metadata API. #29
• extract current sled implementation into separate MetaStore trait and implementation. Keep in mind to support other DBs as well traits for the MetaTree and MetaStore #35
• refactor store_bytes and delete_object refactor meta part 2 #36
• Assess fjall assesment: replace Sled with fjall.rs #11
• Refactor the MetaStore trait to fully support multiple DBs Initial fjall implementation #37

[iwanbk]

refactor store_bytes and delete_object

In store_bytes and delete_object, the code is mixed between disk and metadata operation.

make it a subtask because it is quite complex.

[iwanbk]

Implementation is done.

Further improvement tracked at #38