Every listmonk instance scans the DB periodically to look for
running campaigns to process. This made running multiple instances of
listmonk impractical as they would all pick up the same running
campaign and process them, resulting in duplicate e-mails.
This commit adds a `--passive` flag to the binary that runs listmonk
in a "passive" mode where campaign processing is disabled. This allows
multiple instances of listmonk to be run to handle different kinds of
requests if there is a requirement (scale/traffic?). It is important
to note that there should only be one non-passive instance running at
any given time. If distributed campaign processing is ever considered,
this will change.
11010393d8