Include epoch node in seqs and replacement logic

The {Seq, UUID, Node} triplet uniquely identifies an update in a
cluster.  Including the node that originally accepted the update in
sequences allows us to defend against byzantine cases of shards moving
back and forth between the nodes in a cluster.

This patch updates the sequence generation to include the full triplet
in each element of the _changes sequence.  It also uses the epoch node
instead of the node currently hosting the shard when determing the safe
replacement sequence.

BugzID: 27193