pgsql: Back-patch Heikki's fix to make

Log Message:
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Back-patch Heikki's fix to make TransactionIdIsCurrentTransactionId() use
binary search instead of linear search when checking child-transaction XIDs.
Per example from Robert Treat, the speed of TransactionIdIsCurrentTransactionId
is significantly more important in 8.3 than it was in prior releases, so
this seems worth taking back-patching risk for.

Tags:
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REL8_3_STABLE

Modified Files:
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pgsql/src/backend/access/transam:
twophase.c (r1.39 -> r1.39.2.1)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/twophase.c?r1=1.39&r2=1.39.2.1)
xact.c (r1.257.2.1 -> r1.257.2.2)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/backend/access/transam/xact.c?r1=1.257.2.1&r2=1.257.2.2)