Offload the creation and configuration of the connection object to the
respective worker thread / process in svnserve.

* subversion/svnserve/svnserve.c
  (serve_socket): simple wrapper around serve(), adding connection init
  (serve_thread_t): pass a socket instead of a connection to the thread
  (serve_thread): call the new wrapper
  (main): use the new wrapper to offload work from the main() loop

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1523488 13f79535-47bb-0310-9956-ffa450edef68