pgsql: Add Asserts to verify that catalog cache keys are unique and not

Add Asserts to verify that catalog cache keys are unique and not null.

The catcache code is effectively assuming this already, so let's insist
that the catalog and index are actually declared that way.

Having done that, the comments in indexing.h about non-unique indexes
not being used for catcaches are completely redundant not just mostly so;
and we didn't have such a comment for every such index anyway. So let's
get rid of them.

Per discussion of whether we should identify primary keys for catalogs.
We might or might not take that further step, but this change in itself
will allow quicker detection of misdeclared catcaches, so it seems worth
doing in any case.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8b38a538c0aa5a432dacd90f10805dc667a3d1a0

Modified Files
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src/backend/utils/cache/catcache.c | 18 +++++++++++++++++-
src/include/catalog/indexing.h | 16 +---------------
2 files changed, 18 insertions(+), 16 deletions(-)