mutable files: expose test-and-set operations to HTTP clients

[daira]

Split from ticket:413#comment:2

• Dirnode operations could accept a new argument which provides test-and-set semantics, perhaps "if-old-version-is=XYZ". The webapi protocol would only apply the requested changes if the "current best version" is also described as XYZ
• The human-oriented "wui" could acquire if-old-version-is= arguments on all the buttons it provides. If the directory has been changed between the time the directory page was rendered and the time someone clicks a button, the operation would be rejected with a note encouraging the user to reload the page (and thus get the latest version).

Both mutable files and directories could have a control like this.