Reintegrate the 1.6.x-issue3506-init_once branch:

 * r934599, r934603
   Fix a concurrency issue in the FSFS rep-cache code.  This indirectly
   concerns issue #3506.
   Justification:
     Thread safety.
   Thread:
     http://article.gmane.org/gmane.comp.version-control.subversion.devel/119483
   Notes:
     r934599 updates the svn_atomic__init_once() interface.
     r934603 is the actual fix.
   Notes:
     Depends on the r879966 group, which does not merge cleanly.
   Branch:
     ^/subversion/branches/1.6.x-issue3506-init_once
     (based on ^/subversion/branches/1.6.x-issue3506)
   Votes:
     +1: danielsh, hwright, philip


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.6.x@955687 13f79535-47bb-0310-9956-ffa450edef68