Streaming to/from NOW misbehaves for inactive partitions

When the user requests "now" as a stream endpoint, the client asks each node for its active seqnos. Vbuckets that aren't active on any nodes are currently skipped, and as a result, "stream to now" degrades to "stream to infinity." Likewise, "stream from now" degrades to "stream from beginning."

Suggested fix: retry `getSeqnos()` until every vbucket is active.