WIP Join Removal

As discussed on 26 June, "Join Removal/Vertical Partitioning", here's a
patch to remove joins in certain circumstances.

Tested and blind reviewed, but this is complex and subtle enough I
welcome and expect your comments on corner cases and missed
complications. (Lord knows, I've been down a few blind alleys writing it
to date...)

Patch works, but there's a bit I haven't finished yet - checking unique
indexes. So patch is marked WIP for now. Depending upon how long
commitfest lasts I may have a fully working version. (Yes, I know its
only 1 hours work, but haven't worked out how yet...)

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 Simon Riggs           www.2ndQuadrant.com
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