Pause and Resume consumer

[scottf]

Resolves https://github.com/nats-io/nats.net/issues/867

Adds support for pausing and resuming consumers.

• The IJetStreamManagement now has methods to jsm.PauseConsumer(stream, consumer, pauseUntil), as well as jsm.ResumeConsumer(stream, consumer).
• An example for the above is also added to src/examples.
• ConsumerConfiguration/builder now has support for setting pauseUntil as well. Allowing to set it during consumer creation.