Replace ServerDescriptor#getHiddenServiceDirVersions with ServerDescriptor#isHiddenServiceDir

Tor has never supported versions in the hidden-service-dir descriptor line. See the recently closed legacy/trac#25284 (moved).

We should probably reflect this by replacing ServerDescriptor#getHiddenServiceDirVersions with ServerDescriptor#isHiddenServiceDir.

I'm posting a branch in a minute.