lbcdblocknotify: support multiple stratum server

[peterpan0708]

support notify to multiple stratum servers

[roylee17]

LGTM.

Ideally, we'd like to fan out with multiple stratum connections(or clients), not multiple bridges.
But the current codebase is not friendly to extend it that way.

So I think we can have this working feature merged now and update it in future iterations.
A minor preference is to update the stratum flag to a list style as shown in those inline comments.

This has been requested by multiple pools. So, thanks a lot for the contribution :-)