perf: avoid loading and storing full `Account`s from and to DB

[drahnr]

Current we construct a full struct Account with all details, which queries multiple tables, while we only update a few elements, and hence don't need to fetch the full account. This make 250ms of the full query.

[bobbinth]

Copying from #1483 (comment):

As mentioned in #1481 (comment), we should also move away from the general strategy of reading in the full account from the database, applying delta to it, and then saving it back.