Create 1.6 backport branch

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.6.x-r1443929@1443951 13f79535-47bb-0310-9956-ffa450edef68
On 1.6.x-r1443929 branch: merge r1443929 from trunk and resolve conflicts.

git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.6.x-r1443929@1443954 13f79535-47bb-0310-9956-ffa450edef68
Reintegrate the 1.6.x-r1443929 branch:

 * r1443929
   Introduce a subpool.
   Justifcation:
     Better server memory use.
   Branch:
     ^/subversion/branches/1.6.x-r1443929
   Votes:
     +1: philip, stefan2, breser


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/1.6.x@1462310 13f79535-47bb-0310-9956-ffa450edef68