support collision detection

Scenario:

• User A retrieves issue 1 using remote API
• User B then updates description on Issue 1 using WebUI
• User A also makes changes to the description (without synchronizing the issue first) and sends the updated issue using remote API
  -->
• The issue will have the description from User A
• User A will have no indication that he actually overwrote the description he never saw
• The tool (in this case Mylyn) is not able to determine this collision (without doing expensive checks before each submitting)

It would be great if the posting of an updated issue data would trigger some collision detection on the server and, if one is deteched, fail with a useful message so the tooling can present failure and solution to the user.