Feedback refactor: BlockStore restructure

[matheus23]

Some feedback from dig:

• Adopting bytes::Bytes for BlockStore is not a terrible idea. It's even no_std.
• IpldCodec is pulling in a perhaps unnecessarily heavy dependency on libipld. What about making that simply a u64?
• put_block in practice is often taking &self instead of &mut self, since most people put concurrency primitives inside of their blockstore.

cc @organizedgrime @laudiacay because I know you were looking at blockstore stuff too.

[laudiacay]

i agree with point 2 & 3
don't have an opinion on 1