This separates the JetStream interface into two smaller interfaces:
A JetStream interface which is for producing and consuming messages
and a JetStreamManager interface for creating streams/consumers.
This also adds a new interface that is the compound of both called
JetStreamContext and is the one that is being returned when calling
`nc.JetStream()`.
This change allows to opt-in to the behaviors provided by the
JetStreamContext as needed, for example:
```go
// Can be used to produce/consume messages, but not for creating streams
var js nats.JetStream
js, err = nc.JetStream()
// Can be used for managing streams/consumers
var jsm nats.JetStreamManager
js, err = nc.JetStream()
// Can still be used to both create streams and publish
js, err := nc.JetStream()
```
Signed-off-by: Waldemar Quevedo <wally@synadia.com>
