pgsql: Expose an API for calculating catcache hash values.

Expose an API for calculating catcache hash values.

Now that cache invalidation callbacks get only a hash value, and not a
tuple TID (per commits 632ae6829f7abda34e15082c91d9dfb3fc0f298b and
b5282aa893e565b7844f8237462cb843438cdd5e), the only way they can restrict
what they invalidate is to know what the hash values mean.  setrefs.c was
doing this via a hard-wired assumption but that seems pretty grotty, and
it'll only get worse as more cases come up.  So let's expose a calculation
function that takes the same parameters as SearchSysCache.  Per complaint
from Marko Kreen.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/d4bf3c9c94305e692349fb6fe0c67e483b72ae87

Modified Files
--------------
src/backend/optimizer/plan/setrefs.c |   10 ++------
src/backend/utils/cache/catcache.c   |   40 ++++++++++++++++++++++++++++++++++
src/backend/utils/cache/syscache.c   |   24 ++++++++++++++++++++
src/include/utils/catcache.h         |    4 +++
src/include/utils/syscache.h         |   12 ++++++++++
5 files changed, 83 insertions(+), 7 deletions(-)