dendro v0.3.4

Added

• A writer heartbeat. sources gains two nullable columns, heartbeat and
  heartbeat_interval_ns (no schema version change, FORMAT.md §3.1 and §8).
  The writer sets the interval when it adds or resumes a source and bumps
  the heartbeat of every source it has open every HEARTBEAT_INTERVAL
  (5 s), whether or not it has rows to commit, until the source is
  finalized. A writer that reopens an older archive adds the columns.
  Copies carry both values unchanged.
• SourceRow::heartbeat and SourceRow::heartbeat_interval_ns, and
  archive::HeartbeatWatch, which turns successive reads of a source into a
  WriterState: Complete, Live, Stopped (the heartbeat has not
  changed for STOPPED_AFTER_INTERVALS = 3 intervals: a killed writer or a
  copy) or Unknown (no heartbeat).
• writer::HEARTBEAT_INTERVAL, archive::STOPPED_AFTER_INTERVALS,
  Transaction::set_heartbeat, Transaction::set_heartbeat_interval,
  Transaction::beat and ArchiveMut::ensure_heartbeat_columns.
  WriterState is #[non_exhaustive]. A resumed source continues its
  heartbeat count; an interval of zero or less reads as Unknown.
• Writer::create_beating_every (feature test-support) chooses the
  heartbeat interval.