Refactor upsert_accounts to reduce database round-trips

[SantiagoPittella]

Not for this PR, but this method has gotten pretty complicated with multiple requests to the database being made. Let's create an issue to refactor it so that we issue only one read and one write request during this method (I think this should be enough).

Originally posted by @bobbinth in #1452 (comment)

upsert_accounts currently issues multiple database queries per account in a loop (reads for created_at_block and existing account details, writes for codes, accounts, storage maps, and vault assets).
Refactor to batch all reads into one query and all writes into one batch operation to improve performance and simplify the code.

[bobbinth]

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.

[bobbinth]

Collapsing this into #1538.