provide external method to trigger exports

[schelcj]

Is your feature request related to a problem? Please describe.
There are times where we need to reseed an internal mirror for fan out where triggering exports is problematic with no means to force an export.

Describe the solution you'd like
An external script to trigger an export run optionally allowing for specifying database to export. Alternatively, responding to signals to trigger export of all databases e.g. sending SIGUSR1 starts the export process for all databases.

Describe alternatives you've considered
The only alternative at this time is to lower the sources.{name}.export_timer value, send a HUP, wait for the export to finish and then reset the timer value.

Additional context
Add any other context or screenshots about the feature request here.