pgsql: Perform a lot more sanity checks when freezing tuples.

Perform a lot more sanity checks when freezing tuples.

The previous commit has shown that the sanity checks around freezing
aren't strong enough. Strengthening them seems especially important
because the existance of the bug has caused corruption that we don't
want to make even worse during future vacuum cycles.

The errors are emitted with ereport rather than elog, despite being
"should never happen" messages, so a proper error code is emitted. To
avoid superflous translations, mark messages as internal.

Author: Andres Freund and Alvaro Herrera
Reviewed-By: Alvaro Herrera, Michael Paquier
Discussion: https://postgr.es/m/20171102112019.33wb7g5wp4zpjelu@alap3.anarazel.de
Backpatch: 9.3-

Branch
------
REL9_3_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/4800f16a7ad08fe4145cdbbfedb9f63fdc282e28

Modified Files
--------------
src/backend/access/heap/heapam.c | 133 ++++++++++++++++++++++++++++------
src/backend/access/heap/rewriteheap.c | 5 +-
src/backend/commands/vacuumlazy.c | 15 +++-
src/include/access/heapam.h | 5 +-
src/include/access/heapam_xlog.h | 2 +
5 files changed, 132 insertions(+), 28 deletions(-)