Add an option "http-pipelining" to the servers configuration, so that a user
can disable HTTP pipelining in case that causes problems, e.g. during SSL
renegotiation triggered by the server to request a client certificate.

* subversion/include/svn_config.h
  (SVN_CONFIG_OPTION_HTTP_PIPELINING): New boolean config option.

* subversion/libsvn_ra_serf/ra_serf.h
  (struct svn_ra_serf__session_t): New member variable http_pipelining.

* subversion/libsvn_ra_serf/serf.c
  (load_config): Load the value of the new option from the servers file. If not
       set, use 'HTTP pipelining is enabled' by default.
  (svn_ra_serf__open,
   ra_serf_dup_session): Set the max. nr. of outstanding requests to 1
       if HTTP pipelining is disabled.

* subversion/libsvn_ra_serf/update.c
  (open_connection_if_needed): Set the max. nr. of outstanding requests to 1
       if HTTP pipelining is disabled.

* subversion/libsvn_subr/config_file.c
  (svn_config_ensure): Add the 'http-pipelining' option in the comment section
       of the initial servers file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1619072 13f79535-47bb-0310-9956-ffa450edef68