[krbdev.mit.edu #1784] replay cache performance sucks

[Ken Raeburn via RT]

The performance of our replay cache code is rather poor.  Even with a
replay cache held open, it can be rather slow at times.  My testing
shows highly variable results, from over 1000 requests processed in a
second to under 40.  If an expunge is triggered, the performance gets
really bad.

jik's suggestion of skipping the fsync call might be reasonable, but I
suspect there are other ways to speed it up that reduce actual CPU time
used, without affecting the consistency of the on-disk version.