[issue2551304] Implement collision detection (lost edit prevention) for XML-RPC

New submission from John Rouillard:

It looks like the XML-RPC endpoint doesn't have collision detection. This can lead to lost
edits.

Since xml-rpc is done over http, if-match and etag headers should work to prevent this.
However any existing clients will have to be updated to use it.

I don't see anything in the xmlrpc spec that is designed to prevent this.

Possible sequence that results in lost data:


  user 1 xmlrpc display issue1 status is open
  user 2 xmlrpc display issue1 status is open
  user 2 modifies status to closed via set call
  user 1 modifies status to new via set call

user2's changes are lost.

----------
components: API
messages: 7872
nosy: rouilj
severity: normal
status: new
title: Implement collision detection (lost edit prevention) for XML-RPC
type: behavior

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