editor: cope with slightly out-of-order editor drives As of Subversion 1.8, ra_serf replaces ra_neon as the only http remote access method. ra_serf does lots of clever network things, but as a side effect violates some of the legacy editor API. It turns out we were only barely using those guarantees, so it was straightforward to relax our constraints and work with serf. Prior to this change, attempting to use ra_serf would almost certainly result in an exception due to an out-of-order editor drive. See https://svn.apache.org/repos/asf/subversion/trunk/notes/api-errata/1.7/ra001.txt for more details of the Subversion change.