Revert "Drop unnamed portal immediately after execution to completion"

Revert "Drop unnamed portal immediately after execution to completion"

This reverts commit 1fd981f05369, based on concerns that the logging
improvements do not justify the protocol breakage of dropping an unnamed
portal once its execution has completed.

It seems unlikely that one would try to send an execute or describe
message after the portal has been used, but if they do such
post-completion messages would not be able to process as the previous
versions.  Let's revert this change for now so as we keep compatibility
and consider a different solution.

The tests added by 76bba033128a track the pre-1fd981f05369 behavior, and
are still valid.

Discussion: https://postgr.es/m/CA+TgmoYFJyJNQw3RT7veO3M2BWRE9Aw4hprC5rOcawHZti-f8g@mail.gmail.com