Use smarter seqs when original shard is down/gone

If a shard that contributes to a sequence a) is on a down node or b) is
no longer part of the shard set we can provide enough detail to the
replacement workers to allow them to choose a smarter replacement
sequence.

This patch does not cover the case where the shard that originally
contributed to the sequence is alive but crashes before initializing the
stream to the coordinator.  In that case we're still firing up
replacement workers and asking them to stream from zero.  The
stream_start semantics make reworking that bit a tad trickier.

BugzID: 22698