Fix unit test failure with older versions of python.


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@12040 e27351fd-9f3e-4f54-a53b-843176b1656c
Initial work on cross-pod sharing. This supports the round-tripping of invites across pods with appropriate
bind table entries so that shared collections appear on the sharee side. Access to actual data in the shared
resource is the next thing to be done.


git-svn-id: https://svn.calendarserver.org/repository/calendarserver@12041 e27351fd-9f3e-4f54-a53b-843176b1656c