Fix hot upgrade for new since values

Continuous changes feeds never move to the new version of fabric_view_changes, instead endlessly looping between keep_sending_changes and pack_seqs (among others). As a consequence, the new since value format of {Seq, Uuid} was being passed directly to lists:sum, and crashing.

This patch (largely by @davisp) co-opts a previous deployment that included an anticipatory change for this upgrade which expects a 3-tuple. To trick the system into upgrading smoothly, we send an ignored third term (the empty list, which encodes to a single byte) to be removed in a future release.

This has been SO much fun.