If the passed in xid is the current top transaction, we can do a fast
check and exit early. This should work well for the current heap but
also works very well for proposed AMs that don't use a separate xid
for subtransactions.
Author: Ashwin Agrawal, based on a suggestion from Andres Freund
Reviewed-by: Thomas Munro
Discussion: https://postgr.es/m/CALfoeiv0k3hkEb3Oqk%3DziWqtyk2Jys1UOK5hwRBNeANT_yX%2Bng%40mail.gmail.com
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