pgsql: Refactor GIN's handling of duplicate search entries.

Refactor GIN's handling of duplicate search entries.

The original coding could combine duplicate entries only when they
originated from the same qual condition. In particular it could not
combine cases where multiple qual conditions all give rise to full-index
scan requests, which is an expensive case well worth optimizing. Refactor
so that duplicates are recognized across all the quals.

Branch
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master

Details
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http://git.postgresql.org/gitweb?p=postgresql.git;a=commitdiff;h=56a57473a999b0497e63bde3e303beda5a3c0ff3

Modified Files
--------------
src/backend/access/gin/ginget.c | 419 +++++++++++++++++++-------------------
src/backend/access/gin/ginscan.c | 294 +++++++++++++++------------
src/include/access/gin_private.h | 52 +++--
3 files changed, 402 insertions(+), 363 deletions(-)