Fix bug with accessing to temporary tables of other sessions

Hi,

During previous commitfest this topic already has been discussed
within the "Forbid to DROP temp tables of other sessions" thread [1].
Unfortunately its name doesn't reflect the real problem, so I decided
to start a new thread, as David G. Johnston advised.

Here are the summary results of the discussion [1] :
The superuser is only allowed to DROP temporary relations of other
sessions. Other commands (like SELECT, INSERT, UPDATE, DELETE ...)
must be forbidden to him. Error message for this case will look like
this : `could not access temporary relations of other sessions`.
For now, superuser still can specify such operations because of a bug
in the code that mistakenly recognizes other session's temp table as
permanent (I've covered this topic in more detail in [2]). Attached
patch fixes this bug (targeted on
b51f86e49a7f119004c0ce5d0be89cdf98309141).

Opened issue:
Not everyone liked the idea that table's persistence can be assigned
to table during makeRangeVarXXX calls (inside gram.y).
My opinion - `As far as "pg_temp_" prefix is reserved by the postgres
kernel, we can definitely say that we have encountered a temporary
table when we see this prefix.`

I will be glad to hear your opinion.

--
Best regards,
Daniil Davydov

[1] https://www.postgresql.org/message-id/CAJDiXgj72Axj0d4ojKdRWG_rnkfs4uWY414NL%3D15sCvh7-9rwg%40mail.gmail.com
[2] https://www.postgresql.org/message-id/CAJDiXgj%2B5UKLWSUT5605rJhuw438NmEKecvhFAF2nnrMsgGK3w%40mail.gmail.com