Block replication messages if not replicating

It's true, and just because we're not doing anything about it at the current moment, doesn't mean we shouldn't.

The way to go for votes and block messages is to see if a message of type `m` about round `r` was sent to us from node `n`. We should not be getting the same message type from a node for the same round more than once.

We can have an in-memory data structure for that.

The reason I commented, is that in replication, it's even easier to block it because if a node isn't actively replicating, it shouldn't be getting replication responses.

I think it's fairly simple to add it in the current PR, but we can also add it in a later PR if you feel this PR is too large.

_Originally posted by @yacovm in https://github.com/ava-labs/Simplex/pull/302#discussion_r2581439623_