Fix stonk deadlock by adding missing select_for_update

[interru]

Rules

For my solution I set myself the following additional rules:

• I have to use the functions which are marked as "Do not change".
• I have to treat the functions as black boxes.
• I have to assume that there are additional transactions I don't know about.
• I only focus on the problem described in the README.

I set these rules because otherwise my preferred solution would be to use
bulk updates. But based on the context I doubt that this is what you want
to see here.

Solutions

Potential ways to solve the database deadlock:

1. Remove the atomic.

   Every stonk gets updated in a seperate transaction.

   Problem: This only works if we assume that stonks won't get deleted
   and is a really dirty fix which could cause inconsistent states.

2. Update in the same order

   Deadlocks only happen if two or multiple updates (locks) in transactions are
   interleaving and therefore blocking each other.
   If the updates are ordered and there is an overlap one transaction will
   secure the row level lock first and because of the ordering
   the rest of the data is unlocked. Resolving the problem in this case.

   Problem: This only works if we assume that every transaction does ordered
   updates.

3. Select for update

   Tell the Database that we want to update the selected data.
   This will (on query execution) create a row level lock for every
   stonk we fetch. If there is already a lock held by another transaction
   for a row the query blocks until the lock is released.

I prefer the select for update solution in this case because it most precisely shows the
intent and doesn't rely on assumptions.