the sequence number of the message
Terminology: a source sends a message to a target.
Example sources are Tasks or Contexts. Example targets are Drivers.
The method guarantees that sequence numbers increase strictly monotonically per message source.
So numbers of messages from different sources should not be compared to each other.
Clients of this method must not assume any particular method implementation
because it may change in the future.
The current implementation returns the timestamp of a heartbeat that contained the message.
A heartbeat may contain several messages; all such message will get the same sequence number.
A source can attach only one message to a single heartbeat, so the strict sequence number monotonicity
per source is guaranteed.