There were a series of errors that could result from commit ID conflicts

between review requests and drafts. There were a couple scenarios in

which a conflict could arrise:
* Updating a draft with a commit ID and then posting a new review

  request with the same commit ID.
* Updating two drafts with the same commit ID and trying to publish

  both.
A lot of these problems were due to us relying on database-level

validation for conflict reporting, and not factoring in drafts at all.
We now check for both review requests and drafts with the provided

commit ID before allowing any operations that would result in commit ID

assignment to either a review request or its draft. For the case of

creating a new review request, we do this regardless of whether the

create_from_commit_id flag is set.

All unit tests pass.
Tested the following scenarios:
1) Posted a review request with a commit ID. Tried posting another with the same ID. (Neither was published, but commit IDs stored in the ReviewRequest). The second failed with the expected error.
2) Posted a review request with a commit ID. Updated it with a new commit ID. Tried posting a new review request with the same new commit ID. It failed with the expected error.
3) Posted a review request with a new commit ID. Posted a second review request with a different commit ID. Tried updating both with a new commit ID. It failed with the expected error.
4) Posted a review request with a commit ID. Successfully posted another review request with a different commit ID.
5) Posted a review request with a commit ID. Updated with a different commit ID.