pgsql: Allow units to be specified in relation option setting value.

Allow units to be specified in relation option setting value.

This introduces an infrastructure which allows us to specify the units
like ms (milliseconds) in integer relation option, like GUC parameter.
Currently only autovacuum_vacuum_cost_delay reloption can accept
the units.

Reviewed by Michael Paquier

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/e23014f3d40f7d2c23bc97207fd28efbe5ba102b

Modified Files
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src/backend/access/common/reloptions.c    |   40 ++++++++++++++++-------------
src/include/access/reloptions.h           |    3 ++-
src/test/regress/expected/alter_table.out |   14 ++++++++++
src/test/regress/sql/alter_table.sql      |    6 +++++
4 files changed, 44 insertions(+), 19 deletions(-)