Unable to cleanly close a consumer without unsubscribing from a topic

[klaatu01]

The current behaviour of TopicConsumer::close also unsubscribes from a topic.

In my organisation, we use pulsar as an event broker between bounded contexts, our topics are persistent, and subscriptions are meant for long-term consumption and should outlive the clients consuming from them.

After digging through both the GoLang client and Java client it seems like neither of these perform and unsubscription on close.

I can make a PR to either fix this or add a new function for closing without unsubscribing.

[tisonkun]

Closed by #278.

Releasing 6.0.0 for this one..

[tisonkun]

Released as https://crates.io/crates/pulsar/6.0.0. Thank you @klaatu01!