On the authzperf branch:  (Almost) constant time insertion of ACLs
into the filtered data model.

The idea is simple.  We remember the segments of the previous rule
and the corresponding filtered tree node.  At the beginning of the
next lookup, we can skip the common prefix.  Due to the full model
being pre-sorted, we will only have to do an average of ~1 segment
lookup for a "dense" set of ACLs. 

* subversion/libsvn_repos/authz.c
  (node_segment_pair_t,
   construction_context_t): New data structure to hold the current
                            lookup / path walk.
  (create_construction_context): New constructor for the new type.
  (insert_path): Remember the lookup steps in the new context object.
  (process_acl): Use the new context object to skip the lookup for
                 the common prefix with the previous ACL.
  (create_user_authz): Initialize and provide the context.


git-svn-id: https://svn.apache.org/repos/asf/subversion/branches/authzperf@1621369 13f79535-47bb-0310-9956-ffa450edef68