Work towards using a vector to store the router shard horizons. Doesn't quite work
Comments and rearranging
Fix a bug/testfailure when one of the shards didn't have any topics and wiped out buffered messages from the other shards
Bulk out get horizon command and add test for it. Doesn't pass yet, since we don't give the horizon back in any way
Allow subscribing using a horizon from a previous call to get_horizon, the test now passes
Tidy up offline retro tests