Use repeatable read for pnl tick generation. (backport #2578)

[mergify]

Changelist

By default all transactions in postgres use the read committed isolation level. This means that any SELECT within a transaction will only see data that was committed before the select was run. However, this means that SELECTs within a transaction done at different times may see different data. This is an issue for the PnL task which gets data from the transfers table and the perpetual_positions table to determine the PnL snapshot of a subaccount, as there can be a race condition between reading the transfers and perpetual positions if ender is updating both tables.
Instead we should use the repeatable read transaction isolation level, that guarantees within a transaction all SELECTs will read the same snapshot of data. We only use this for the read-only transaction in the pnl task to ensure no rollbacks / retries need to be added.

Transaction isolation level reference

Test Plan

Unit tests, running in a deployed environment.

Author/Reviewer Checklist

• If this PR has changes that result in a different app state given the same prior state and transaction list, manually add the state-breaking label.
• If the PR has breaking postgres changes to the indexer add the indexer-postgres-breaking label.
• If this PR isn't state-breaking but has changes that modify behavior in PrepareProposal or ProcessProposal, manually add the label proposal-breaking.
• If this PR is one of many that implement a specific feature, manually label them all feature:[feature-name].
• If you wish to for mergify-bot to automatically create a PR to backport your change to a release branch, manually add the label backport/[branch-name].
• Manually add any of the following labels: refactor, chore, bug.

Summary by CodeRabbit

• New Features

  • Enhanced transaction management for creating PNL ticks, ensuring more consistent data reads.

• Bug Fixes

  • Improved error handling during the fetching of PNL ticks to ensure better logging and recovery.

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This is an automatic backport of pull request #2578 done by [Mergify](https://mergify.com).

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