pgsql: Improve RLS handling in copy.c

Improve RLS handling in copy.c

To avoid a race condition where the relation being COPY'd could be
changed into a view or otherwise modified, keep the original lock
on the relation. Further, fully qualify the relation when building
the query up.

Also remove the poorly thought-out Assert() and check the entire
relationOids list as, post-RLS, there can certainly be multiple
relations involved and the planner does not guarantee their ordering.

Per discussion with Noah and Andres.

Back-patch to 9.5 where RLS was introduced.

Branch
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REL9_5_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/5d179a28fb4c819f3812c40fa7e626b1d3081982

Modified Files
--------------
src/backend/commands/copy.c | 45 +++++++++++++++----------
src/test/regress/expected/rowsecurity.out | 50 +++++++++++++++++++++++++++-
src/test/regress/sql/rowsecurity.sql | 51 ++++++++++++++++++++++++++++-
3 files changed, 126 insertions(+), 20 deletions(-)