refactor: change account ID SQL's backing type

[tomyrd]

closes #602

In this PR we change the account ID sql columns from blobs to two integers (u64). This allows us to index only with the prefix (as it uniquely identifies accounts) which is faster than indexing with the whole ID.

In this implementation I used two integers in tables where the full account ID was needed and only prefix when it was used as a foreign key.

[tomyrd]

I wanted to ask about this part:

used two integers in tables where the full account ID was needed

This is not technically necessary as the suffix may be stored only in the accounts table and retreived using a JOIN but I wasn't sure how this would affect the performance. If we prefer, I can use only prefix and get the suffix with joins if needed.

[igamigo]

LGTM! I think this approach should work well. I left a small comment, but everything else looks good. Also, not sure but it might be worth mentioning (maybe in a comment on the migration file) why account IDs are broken up into 2, and why the prefix works as a primary key, etc.

[igamigo]

We should add some brief doc comments to this function.

[bobbinth]

I would probably put this PR on-hold for now. Ideally, we'd have some sort of a framework to measure performance impact of optimizations like this before implementing them.