pgsql: Spit items: * Experiment with multi-threaded backend better I/O

Log Message:
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Spit items:

* Experiment with multi-threaded backend better I/O utilization

This would allow a single query to make use of multiple I/O channels
simultaneously. One idea is to create a background reader that can
pre-fetch sequential and index scan pages needed by other backends.
This could be expanded to allow concurrent reads from multiple devices
in a partitioned table.

* Experiment with multi-threaded backend better CPU utilization

This would allow several CPUs to be used for a single query, such as
for sorting or query execution.

Modified Files:
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pgsql/doc:
TODO (r1.2349 -> r1.2350)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/TODO?r1=1.2349&r2=1.2350)
pgsql/doc/src/FAQ:
TODO.html (r1.857 -> r1.858)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/TODO.html?r1=1.857&r2=1.858)