pgsql: Use fuzzy path cost tiebreaking rule in our oldest supported bra

Use fuzzy path cost tiebreaking rule in our oldest supported branches.

We've been doing it that way since 9.2, cf commit 33e99153e93b9acc,
but some recently-added regression test cases are making a few buildfarm
members fail (ie choose the "wrong" plan) in 9.0 and 9.1 due to
platform-specific roundoff differences in cost calculations. To fix,
back-port the patch that made add_path treat cost difference ratios of
less than 1e-10 as equal.

Branch
------
REL9_0_STABLE

Details
-------
http://git.postgresql.org/pg/commitdiff/9d6af7367e7b079c021c285f03128bad531b667e

Modified Files
--------------
src/backend/optimizer/util/pathnode.c | 50 +++++++++++++++++++--------------
1 file changed, 29 insertions(+), 21 deletions(-)

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