Merge the r1468980 group from trunk:

 * r1468980, r1478987, r1502811
   Reduce the number of ra sessions for automatic merge calculations to two.
   Justification:
     10 concurrent ra sessions open to a single server at the same time breaks
     in certain setups (e.g. xinetd svnserve), while we really don't need this
     many. Opening ra session is still a performance killer in all other
     scenarios, so reducing the number of sessions is a good thing.
   Notes:
     Depends on r1502901 and r1502909, to avoid ra-session reuse errors.
     r1468980 is a small performance tweak needed for a clean merge.
     r1478987 removes a common ra session.
     r1502811 avoids all ra session creations via svn_client__mergeinfo_log.
   Votes:
     +1: rhuijben, stefan2, ivan
     +0: breser (I see that the call to
                 svn_ra__assert_mergeinfo_capable_server() was removed from
                 get_mergeinfo() in mergeinfo.c, but not mentioned in commit
                 logs.  Looking at the code it seems that finding out if the
                 server and repo support merginfo requires trying to get some,
                 but don't we need to pretty up the error here like the assert
                 would do for us?)


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.8.x@1523183 13f79535-47bb-0310-9956-ffa450edef68