Consider auto-discover SVNMasterVersion value

http://mid.gmane.org/20130219140939.GA44271@minotaur.apache.org

We should look into having the slave automatically discover the
SVN_VER_MAJOR.SVN_VER_MINOR version of the master, in order to make it
unnecessary to set SVNMasterVersion manually.

It has been noted that the master server might wish not to disclose that
information.  If that is the case we might wish to retain SVNMasterVersion as a
fallback that only needs to be set when the master is (a) an older minor line
than the slave, and (b) doesn't disclose its version [that is not the default
behaviour].

It has also been noted that it would be useful to cache the information (in the
slave server), since it rarely changes.

http://mid.gmane.org/20130219140939.GA44271@minotaur.apache.org