GIN improvements part 1: additional information

Hackers,

Revised version of patch for additional information storage in GIN is
attached. Changes are mostly bug fixes.

Resemble GIN interface changes that this patch introduce.
Patch modifies GIN interface as following:
1) Two arguments are added to extractValue
Datum **addInfo, bool **addInfoIsNull
2) Two arguments are added to consistent
Datum addInfo[], bool addInfoIsNull[]
3) New method config is introduced which returns datatype oid of addtional
information (analogy with SP-GiST config method).

Additionally there is another patch which demonstrates benefits from
additional information storage itself (because it don't accelerate tsearch
itselt). It comes in separate thread.

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With best regards,
Alexander Korotkov.