pgsql: Resolve timing issue with logging locks for Hot Standby.

Resolve timing issue with logging locks for Hot Standby.
We log AccessExclusiveLocks for replay onto standby nodes,
but because of timing issues on ProcArray it is possible to
log a lock that is still held by a just committed transaction
that is very soon to be removed. To avoid any timing issue we
avoid applying locks made by transactions with InvalidXid.

Simon Riggs, bug report Tom Lane, diagnosis Pavan Deolasee

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/8572cc495cd07d4f4a59624d275a75b45340a3b2

Modified Files
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src/backend/storage/ipc/procarray.c |    8 +--
src/backend/storage/ipc/standby.c   |  110 ++++++++++++++++++++++++-----------
src/backend/storage/lmgr/lock.c     |   12 ++++-
src/include/storage/standby.h       |    2 +-
4 files changed, 88 insertions(+), 44 deletions(-)