Don't try to truncate multixact SLRU files in checkpoints done during xlog

Don't try to truncate multixact SLRU files in checkpoints done during xlog
recovery.  In the first place, it doesn't work because slru's
latest_page_number isn't set up yet (this is why we've been hearing reports
of strange "apparent wraparound" log messages during crash recovery, but
only from people who'd managed to advance their next-mxact counters some
considerable distance from 0).  In the second place, it seems a bit unwise
to be throwing away data during crash recovery anwyway.  This latter
consideration convinces me to just disable truncation during recovery,
rather than computing latest_page_number and pushing ahead.