Create reusable mechanism for configuring output sequence numbers

[nigelmegitt]

Currently producer nodes always begin their output sequence at 1, but in general this is not always the best choice. See also #502 (comment)

Allow output sequence numbering strategy to be determined by configuration, with an interface class that provides the facility to fetch a new sequence number, and objects that satisfy that interface where at least:

1. One of them begins at sequence number n, default 1.
2. One of them uses a mapping of the current time (perhaps from a reference clock) to the sequence number.