pgsql: Extend relations multiple blocks at a time to improve scalabilit

Extend relations multiple blocks at a time to improve scalability.

Contention on the relation extension lock can become quite fierce when
multiple processes are inserting data into the same relation at the same
time at a high rate.  Experimentation shows the extending the relation
multiple blocks at a time improves scalability.

Dilip Kumar, reviewed by Petr Jelinek, Amit Kapila, and me.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/719c84c1be51f3d3fe6049b77ddbaa0c4b58a9a9

Modified Files
--------------
src/backend/access/heap/hio.c             | 112 +++++++++++++++++++++++++++++-
src/backend/storage/freespace/freespace.c |  81 +++++++++++++++++++++
src/backend/storage/lmgr/lmgr.c           |  35 ++++++++++
src/backend/storage/lmgr/lock.c           |  37 ++++++++++
src/include/storage/freespace.h           |   4 ++
src/include/storage/lmgr.h                |   3 +
src/include/storage/lock.h                |   2 +
7 files changed, 271 insertions(+), 3 deletions(-)