Kluge solution for Alex Pilosov's report of problems with whole-tuple

function arguments in join queries: copy the tuples into
TransactionCommandContext so they don't get recycled too soon.  This is
horrid, but not any worse than 7.0 or before, which also leaked such
tuples until end of query.  A proper fix will require allowing tuple
datums to be physically stored inside larger tuple datums, which opens
up a bunch of issues that can't realistically be solved for 7.1.1.