refactor: store the accountID as bytes or as two separate u64 to improve indexing

[tomyrd]

Similar to what is stated in 0xPolygonMiden/miden-node#602. We currently store it as a string which may hinder indexing and serialization/deserialization.

The ID prefix can be used as the only primary key as it is a unique identifier, and the sufix is just stored to get the full ID when needed.

[bobbinth]

My intuition is that here performance is much less critical than in the node, so storing account ID as a blob (or even as a string) may be much more acceptable.