pgsql: Rearrange storage of data in xl_running_xacts.

Rearrange storage of data in xl_running_xacts.
Previously we stored all xids mixed together.
Now we store top-level xids first, followed
by all subxids. Also skip logging any subxids
if the snapshot is suboverflowed, since there
are potentially large numbers of them and they
are not useful in that case anyway. Has value
in the envisaged design for decoding of WAL.
No planned effect on Hot Standby.

Andres Freund, reviewed by me

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/5c11725867ac3cb06db065f7940143114280649c

Modified Files
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src/backend/access/transam/xlog.c | 2 +
src/backend/storage/ipc/procarray.c | 65 +++++++++++++++++++++++------------
src/backend/storage/ipc/standby.c | 8 +++--
src/include/storage/standby.h | 2 +
4 files changed, 52 insertions(+), 25 deletions(-)