Revision 1611379

Fix Windows redo loop oddity for named atomics.  As it turns out, the redo
loops for file creation and locking don't play very well with files that
get / got deleted.  This is the root cause for the bad ra_serf performance
when revprop caching has been enabled.

With this patch, we simply use the same lock file scheme as for e.g. lock
files in FSFS, i.e. we auto-create an empty lock file and keep it.

* subversion/libsvn_subr/named_atomic.c
  (mutex_t): Don't keep the lock file open while not holding the lock,
             i.e. the mutex is simply the file name and the pool that
             will contain the actual file lock.
  (lock): Auto-create and lock the file as we do in e.g. FSFS.
  (unlock): Simple pool cleanup now released the lock file.
  (delete_lock_file): Drop.
  (svn_atomic_namespace__create): Update mutex initialization code.