pgsql: Derive oldestActiveXid at correct time for Hot Standby.

Derive oldestActiveXid at correct time for Hot Standby.
There was a timing window between when oldestActiveXid was derived
and when it should have been derived that only shows itself under
heavy load. Move code around to ensure correct timing of derivation.
No change to StartupSUBTRANS() code, which is where this failed.

Bug report by Chris Redekop

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/bf70bf4c7177bdd5c6644da65c0ccea47405d5f3

Modified Files
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src/backend/access/transam/xlog.c | 14 +++++++--
src/backend/storage/ipc/procarray.c | 57 +++++++++++++++++++++++++++++++++++
src/backend/storage/ipc/standby.c | 3 +-
src/include/storage/procarray.h | 1 +
src/include/storage/standby.h | 2 +-
5 files changed, 71 insertions(+), 6 deletions(-)