[PATCH 08/14] Store the number of subtransactions in xl_running_xacts separately from toplevel xids

To avoid complicating logic we store both, the toplevel and the subxids, in
->xip, first ->xcnt toplevel ones, and then ->subxcnt subxids.
Also skip logging any subxids if the snapshot is suboverflowed, they aren't
useful in that case anyway.

This allows to make some operations cheaper and it allows faster startup for
the future logical decoding feature because that doesn't care about
subtransactions/suboverflow'edness.
---
 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(-)