Previously the greylist plugin tried to use redis and fell back to DBM. This means that if a system already had an established DBM database, but happened to have Redis running, the existing DBM db would be abandoned for a new Redis DB. This would inevitably lead to more delays for legitimate mail, and possibly lost mail.
This adds a 'redis_server' argument which enables Redis and sets the location of the redis server; if it is not explicitly set, DBM is used instead.
If the redis server is unavailable, rather than failing to start QP, we instead fail to register the plugin.
4f9af75e48