Checks that the sequenced events' stream is gap-free:

• We expect event timestamps to be monotonically increasing
• The sequence should start with previousTimestamp = None.
• For each subsequent event, we expect previousTimestamp to be set to the timestamp of the previous event.
• When a violation is detected, an error is logged and the processing is aborted.

This is normally ensured by the com.digitalasset.canton.sequencing.client.SequencedEventValidator for individual sequencer subscriptions. However, due to aggregating multiple subscriptions from several sequencers up to a threshold, the stream of events emitted by the aggregation may violate monotonicity. This additional monotonicity check ensures that we catch such violations before we pass the events downstream.