bridge: Add fsread1, fswrite1, and fswatch1 payload types.

You get a tag that is at least as recent as the change that triggered the event.  It is easily possible to make a number of quick changes to a file, and then after a while get a number of change events all with the same tag.

Also, of course, by the time you read the file, its content might have changed again, but the fsread channel will then tell you the tag for the actual content that you have reveived.

So I think the semantics are OK here.