fix: Don't mark blob as confirmed if already confirmed

[ian-shim]

Why are these changes needed?

There seems to be a race condition that results in some blobs being encoded/dispersed multiple times.
This affects the centralized retriever at the disperser because duplicate dispersals overwrite/corrupt the metadata from the initial confirmation.
This PR is a temporary solution that makes a confirmed metadata immutable. No additional dispersals would overwrite the existing confirmation metadata. Note that it does it in a way that is very inefficient, adding an extra call to retrieve the metadata for every MarkBlobConfirmed call.

Checks

• I've made sure the lint is passing in this PR.
• I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(

[jianoaix]

lgtm as a stopgap