Only first XLogRecData is visible to rm_desc with WAL_DEBUG

As we all know, when you compile with WAL_DEBUG, and enable wal_debug
GUC, you get output like this in the log for every inserted WAL record:

LOG: INSERT @ 0/5407E578: prev 0/5407E4D0; xid 0; len 32: Standby -
running xacts: nextXid 774 latestCompletedXid 771 oldestRunningXid 772;
2 xacts: 7877929 0

What I did *not* realize before is that the redo routine only gets
passed the first XLogRecData struct in the chain. Because of that, the
"xacts: 7877929 0" information above is garbage. That's probably not the
only rm_desc routine that didn't get the memo.

There are a few alternatives on how to fix that:

1. Do nothing. Add a comment somewhere explaining that rm_redo cannot
safely look at data beyond what's inserted in the first XLogRecData.

2. Reconstruct the WAL data from the XLogRecData entries in a palloc'd
buffer, and pass that to rm_redo. That would be fairly expensive, but
you probably don't care about that if you've enabled wal_debug.

3. Remove the feature altogether, so that enabling wal_debug doesn't
cause all insertions to be logged anymore (no changes to the logging
during replay). It's a lot less interesting now that we have pg_xlogdump.

Thoughts?

- Heikki