ntdll: Use lockfree implementation for get_cached_fd. (try 3)

[Sebastian Lackner]

Thanks for all the feedback @ Ken, Daniel, Bruno and all the other
people I talked to.

Changes in try 3:

* Although the fd_cache_epoch was slightly faster in my tests (-> see
plots in mail for try 1) and should also work on PPC, its probably
better to choose a method which is easier to verify, and where we can be
sure, that it doesn't break with the next gcc release. ;) In this patch
I switched back to the old approach using interlocked_cmpxchg64 to
set/get both fd and associated attributes at the same time.

* Although used in some other places, its probably better to avoid
assert() for stuff, which should never be optimized away.

---
 dlls/ntdll/server.c |   50
++++++++++++++++++++++++++++++++++++++++++++++----
 1 file changed, 46 insertions(+), 4 deletions(-)


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