I believe we can retroactively add compatibility with pre-1.11 ipropx 
dumps, by trying the 1.11 policy format and then the 1.8 format.

We can't retroactively add compatibility in the old-slave direction, 
but compatibility in that direction is problematic anyway, as there is 
no easy way to pass a per-slave flag to kdb5_util dump to generate an 
old-format dump on the master.  (With traditional kprop, the admin is 
responsible for writing a cron job which generates the dump and invokes 
kprop for each slave, and can generate older-format dumps for older 
slaves in the job.)

Regardless of what we do, we need to document this issue where we talk 
about upgrades.