Shard listener manager

[adlrocha]

Every peer needs to listen at least to its shard, the root chain, and its shard parent chain pubsub topic. Additional, to support cross-shard transactions, peers should have a way to proactively be able to join other shard pubsub topic to share and listen to messages.

We need to handle conveniently the pubsub topics a client is listening to when they join/leave a shard.