MergeRequests::MergeService does not handle errors that occur after merge

As described in https://gitlab.com/gitlab-org/gitlab-ce/issues/23505#note_24909714, MergeRequests::MergeService does not handle errors that can occur in MergeRequests::MergeService#after_merge (after a merge commit is created).

Things to consider:

• log and rescue errors occurring after merge
• do not retry MergeWorker because the task is not transactional
• do not perform a merge if the merge request has merge_commit_sha set
• reorder operations in MergeRequests::PostMergeService so that changing status to "merged" executes before operations that may result in HTTP calls to external services (MergeRequests::PostMergeService#close_issues)