Add temporary debugging assertion, in 9.4 branch only.

Buildfarm member okapi has been failing the multiple-cic isolation
test for months now, but only in 9.4.  To narrow down the possible
causes, add an Assert testing that CREATE INDEX CONCURRENTLY is
advertising zero xmin before waiting for other transactions to end.

I'm not sure that this would hold in general, so this assertion isn't
meant to get released, but it passes all 9.4 regression tests for me.
Will revert once we see how okapi responds.