Auto-unsubscribe options

[mtmk]

Users should be able to automatically unsubscribe:

• After a certain number of messages (max-msgs)
• After a certain time period (timeout)
• Time period after the previous message was received (idle timeout)
• Also, unsubscribe explicitly (before Dispose)

First scenario (max-msgs) can be implemented using UNSUB max_msgs:

• SUB then immediately UNSUB with max_msgs specified
• Keep a message count for subscription
• When count == max_msgs signal user code (e.g. end iterators) and remove subscription
• When reconnecting UNSUB with max_msgs - count

Timeouts would complete iterators and unsubscribe.

If combination of the options is used then whichever happens first wins and cancel others.

• add Unsubscribe() method on NatsSub
• add MaxMsgs on SubOpts
• add Timeout on SubOpts
• add IdleTimeout on SubOpts

[Jarema]

In Rust and Go clients, there are those methods:

Rust: sub.unsubscribe_after(max: u64) -> Result<(), UnsubscribeError>
Go: func (s *Subscription) AutoUnsubscribe(max int) error

So both are called on active subscription.
Both are also counting number of messages since the start of the subscription, and when max is already reched when setting UNSUB, it will immediately close the subscription.

If not, it will check the max internally at every message, and close subscription when counter reaches max.

For re-subscribing, you can still maintain the subscription state on the client side, and just resubscribe with the server.

[mtmk]

Currently we don't have an explicit unsubscribe method, it's done during subscription dispose. More natural way might be to add maxMsgs field to subscription options in our case.