Minor speed-up to the str->time conversion function that is
used e.g. in our network layers.

* subversion/libsvn_subr/time.c
  (svn_time_from_cstring): use SVN's own, faster atol variant

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1533387 13f79535-47bb-0310-9956-ffa450edef68