jobs: remove WithTxn

[ajwerner]

There is a WithTxn() method which will associate (and store) a *kv.Txn with the *jobs.Job object. That txn will be used in all of the methods which update the job state in the KV store. This is unfortunate on several fronts. Firstly, it's not always clear which methods operate on in-memory state vs on the key-value store because nothing takes a txn. Secondly, if you don't clear the transaction, then later you can run into trouble using a potentially already committed transaction.

Methods which write to the KV-store should take a transaction.

[blathers-crl]

Hi @ajwerner, please add a C-ategory label to your issue. Check out the label system docs.

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