Re: Fix slot's xmin advancement and subxact's lost snapshots indecoding.

Pushed this to 9.4 - master after some more tinkering.

It occurred to me that it might be better to have
ReorderBufferSetBaseSnapshot do the IncrRefCount instead of expecting
caller to do it.  But I wouldn't backpatch that change, so I refrained.

Thanks for the patch.

-- 
Álvaro Herrera                https://www.2ndQuadrant.com/
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