pgsql: Remove O(N^2) performance issue with multiple SAVEPOINTs.

Remove O(N^2) performance issue with multiple SAVEPOINTs.
Subtransaction locks now released en masse at main commit, rather than
repeatedly re-scanning for locks as we ascend the nested transaction tree.
Split transaction state TBLOCK_SUBEND into two states, TBLOCK_SUBCOMMIT
and TBLOCK_SUBRELEASE to allow the commit path to be optimised using
the existing code in ResourceOwnerRelease() which appears to have been
intended for this usage, judging from comments therein.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/7cb7122800ec996d4849ce9b4ad3065db19a2aae

Modified Files
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src/backend/access/transam/xact.c |  102 +++++++++++++++++++++++++------------
1 files changed, 70 insertions(+), 32 deletions(-)