add support for detecting mid-air collisions

[Steffen Pingel]

If the same field of a ticket is updated concurrently by two users the later update overrides the previous change without notice. It would be helpful if Trac supported versioning of tickets and would reject an update if the provided data did not match the version currently in the repository. An example of that is Bugzilla's has a notion of mid-air collision which require updating first and re-submitting in case of a conflict.

The original request filed against Mylyn is tracked here:

253932: submitting changes also sends unchanged fields possibly resetting them to old values ​https://bugs.eclipse.org/bugs/show_bug.cgi?id=253932