pgsql: Modify local buffer management to request memory for local

Log Message:
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Modify local buffer management to request memory for local buffers in blocks
of increasing size, instead of one at a time. This reduces the memory
management overhead when num_temp_buffers is large: in the previous coding
we would actually waste 50% of the space used for temp buffers, because aset.c
would round the individual requests up to 16K. Problem noted while studying
a performance issue reported by Steven Flatt.

Back-patch as far as 8.1 --- older versions used few enough local buffers
that the issue isn't significant for them.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql/src/backend/storage/buffer:
localbuf.c (r1.70.2.2 -> r1.70.2.3)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/localbuf.c.diff?r1=1.70.2.2&r2=1.70.2.3)