Plan reporting: Provide get-last-plan queuedaemon command

Plan reporting: Provide get-last-plan queuedaemon command

This allows retrieval, by monitoring clients which are not
participating in the planning queue, of the finished projection, or
the unfinished plan as it was at the time of last restart.

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
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v2: Fix invocation of return-plan-to-client.
    Use data-W.final.pl, not data-W-final.pl, to fit
        with existing .gitignore, and be slightly neater.