Detect new commits in branch to support gitflow model

[wobrschalek]

Problem:
During merge the commit of the merged data should not overwrite any new commits in the target branch.
MMS should detect if there is a commitId mismatch and reject that commit

Proposed solution:
Add commitId as new optional parameter to the endpoint createOrUpdateElements (/projects/{projectId}/refs/{refId}/elements).
If the latest commitId on that branch doesn't match the provided commitId, the server should reject the POST request (for example with "409 Conflict").