client close method about pulsar-rs

[Geal]

so, after testing things out in #73, I believe safely closing the client is tricky, as we spawn a lot of futures that loop on orders from channels:

• the service discovery engine handles DNS lookups (when we look up a topic, it might be handled by a different broker, for which we need to look up an IP)
• the connection manager handles TCP connection creation
• the producer engine receives a request to send a message to a channel, and possibly creates a TopicProducer for it (cf #74)
• the Sender sends messages on a connection
• the Receiver handles incoming messages from a connection

we need to make sure that all of those futures are closed cleanly, and so, properly close all the channels

[Geal]

one engine down (service discovery now does queries directly): 626b55c

[Geal]

same for the connection manager engine: 867dfff

[Geal]

done in 1.0 release