Work around replay cache creation race

Here is the absolute minimum we can do to ease the pain of the rcache creation race (RT #3498).  If we have an A-unlink B-unlink A-open B-open race, we make the losing process (B in this case) retry.  Of course this results in A's file descriptor pointing to an unlinked file, just as if A had lost an A-unlink A-open B-unlink B-open race.  But it avoids spurious failures.