feat: subscription policies

[merklefruit]

As part of #13, we want to add the ability for subscribers to specify the "durability policy" that they want to use.
Here's how that would look like:

• subscriber (ReqSocket or SubSocket) connects to a target, specifying the policy: if they would like to receive messages from the connection point onward, or if they want to also receive cached messages that were already broadcasted earlier than the connection time.
• On the publisher's side there will always be a limit of how many messages to replay for the specific subscriber. Nonetheless, this will be useful for when clients are starting up to make sure they don't lose initial messages.

[quangkeu95]

I can take on this.

[mempirate]

@quangkeu95 this issue is blocked by #13, which we still need to figure out how to implement. Will ping you once we have a better idea of how to move forward!

[quangkeu95]

sure, thanks for letting me know