Cleanup: avoid direct use of ip_posid/ip_blkid

Hello All,

I would like to propose the attached patch which removes all direct
references to ip_posid and ip_blkid members of ItemPointerData struct and
instead use ItemPointerGetOffsetNumber and ItemPointerGetBlockNumber macros
respectively to access these members.

My motivation to do this is to try to free-up some bits from ip_posid
field, given that it can only be maximum 13-bits long. But even without
that, it seems like a good cleanup to me.

One reason why these macros are not always used is because they typically
do assert-validation to ensure ip_posid has a valid value. There are a few
places in code, especially in GIN and also when we are dealing with
user-supplied TIDs when we might get a TID with invalid ip_posid. I've
handled that by defining and using separate macros which skip the
validation. This doesn't seem any worse than what we are already doing.

make check-world with --enable-tap-tests passes.

Comments/suggestions?

Thanks,
Pavan

--
Pavan Deolasee http://www.2ndQuadrant.com/
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