[nexus] Use retryable transactions more extensively

[smklein]

This PR finds spots where we use transaction_async and makes them use transaction_retry_wrapper instead.

This means that under contention, we'll avoid wasting work, and can make use of CockroachDB's automated retry mechanisms.

Additionally, this PR adds a clippy lint to help future usage avoid the "non-retryable" transaction variant.
There are some use cases where avoiding retries is still reasonable:

1. Test-only code
2. Transactions which have truly minimal contention, or which can fail with serialization errors without issue
3. Nested transactions

[bnaecker]

Looks great, thanks for converting these! A few small suggestions, merge at your discretion. Thanks!