In order that people can use formats like #D in #() in the status line

In order that people can use formats like #D in #() in the status line
and not have to wait for an update when they change pane, we allow
commands to run more than once a second if the expanded form
changes. Unfortunately this can mean them being run far too often
(pretty much continually) when multiple clients exist, because some
formats (including #D) will always differ between clients.

To avoid this, give each client its own tree of jobs which means that
the same command will be different instances for each client - similar
to how we have the tag to separate commands for different panes.

GitHub issue 889; test case reported by Paul Johnson.