Check client datacap before getting to publish deals

A user reports that a deal failed at the publish stage with an error message of the form
```
Deal size of 12345 is greater than verifier_cap 1234 for verified client f01234
```

The deal should not reach the publish stage. It should be rejected at the deal verification stage.

This may be caused by a race condition in the way that datacap is checked between different nodes, rather than because it's not being checked at all.

Trace the deal verification and publish paths to find where client datacap is checked. Investigate whether checks can race across nodes, using the reported verifier_cap error as the failure case. Done means an over-cap deal is rejected during verification rather than reaching publish, including when nodes check datacap concurrently.

Written by the indexing model from the issue text.