Better fix for deadlock hazard in CREATE INDEX CONCURRENTLY.

Commit 54eff5311 did not account for the possibility that we'd have
a transaction snapshot due to default_transaction_isolation being
set high enough to require one.  The transaction snapshot is enough
to hold back our advertised xmin and thus risk deadlock anyway.
The only way to get rid of that snap is to start a new transaction,
so let's do that instead.  Also throw in an assert checking that we
really have gotten to a state where no xmin is being advertised.

Back-patch to 9.4, like the previous commit.

Discussion: https://postgr.es/m/CAMkU=1ztk3TpQdcUNbxq93pc80FrXUjpDWLGMeVBDx71GHNwZQ@mail.gmail.com

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