Add async request_timeout

[erebe]

• Add support for request_timeout in async nats

Why:

• If we get disconnected from NATS for reason, as the lib use unblock() under the hood, the application will leak threads waiting for the operation to receive a reply that will never comeback.
• Even if we get back the connection to NATS, those threads are lost in the limbo as they will never be able to receive a reply
• Wrapping async request within a futures::timeout, works but does not kill the underlying threads used by unblock

What:

• The PR exposes the request_timeout function in async-nats

[spacejam]

LGTM, and thank you for updating the changelog and adding a new version! The tests are just failing on a clippy issue that I've already addressed on master.

[spacejam]

0.9.15 has been published which includes your change. Thanks again!

[erebe]

Thank you, good sir :)