pgsql: Allocate local buffers in a context of their own, rather than

Log Message:
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Allocate local buffers in a context of their own, rather than dumping them
into TopMemoryContext. This makes no functional difference, but makes it
easier to see what the space is being used for in MemoryContextStats dumps.
Per a recent example in which I was surprised by the size of TopMemoryContext.

Modified Files:
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pgsql/src/backend/storage/buffer:
localbuf.c (r1.90 -> r1.91)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/storage/buffer/localbuf.c?r1=1.90&r2=1.91)