Extend lbcblocknotify to support custom command and multiple destinations.

[roylee17]

So far, the users of the lbcdblocknotify programs are predominantly mining pools. For reliability, some pools have multiple instances of backend and frontend services that all act upon the notification. In this case, users have to set up multiple instances of lbcdblocknotify

It is desired to have multiple subscribers for the notification where each of them can customize their command and end points.

[roylee17]

Support running a custom command with '--run' seems enough as user can wrap the rest in their custom script.