heap_freeze_tuple locking requirements

heap_freeze_tuple() was apparently designed at one point to cope with
being called with either a shared or exclusive buffer lock. But none
of the current callers call it with a shared lock; they all call it
with an exclusive lock, except for the heap-rewrite code which doesn't
take (or need) a lock at all. But, meanwhile, the comments for that
function claim that vacuum does call it that way, which is incorrect,
and AFAICT has been at least since HOT went in. Attached is a
proposed patch that cleans all of this up by removing support for
calling it with just a shared lock and removing or adjusting various
related comments.

Since this is just dead code removal, I propose to apply this to 9.2.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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